geodesy category report - terra institute category report prepared by ian wilson ... st augustine...

GEODESY CATEGORY REPORT

Prepared by Ian Wilson

DRAFT

Reviewed by Fred Brazier and Charisse Griffith-Charles

2000, February 3

LAND TENURE CENTER University of Wisconsin - Madison

Consultancy Services to

The Government of the

Republic of Trinidad & Tobago

LAND USE POLICY AND ADMINISTRATION PROJECT (LUPAP) LAND SURVEYING COMPONENT
30 Queens Park West, Port of Spain, Trinidad & Tobago. Tel: + 1 (868) 628-2241; Fax + 1 (868) 628-2260; Email: [email protected]

Saved: 2000-02-14 4:39 PM Printed2000-05-09 5:50 AM

LUPAP: Land Surveying Component. GEODESY REPORT

Contents 1. Introduction....................................................................................................3 2. Scope ............................................................................................................3 3. Current Situation in Trinidad and Tobago ......................................................3

3.1 Research....................................................................................................3 3.2 Datums in use ............................................................................................3 3.3 Current Practice .........................................................................................4 3.4 Work on a new Geodetic Infrastructure......................................................5 3.5 Comment....................................................................................................5

4. The need for a geodetic datum......................................................................6 4.1 What is a Geodetic Datum? .......................................................................6 4.2 Why is a datum needed?............................................................................7 4.3 The Effect of GPS ......................................................................................7 4.4 Accessibility................................................................................................8

5. Geodetic ‘tools’ ..............................................................................................8 5.1 Transformations .........................................................................................8 5.2 Geoid Model...............................................................................................8

6. The Future .....................................................................................................9 6.1 The Datum Issue........................................................................................9 6.2 National GPS Network ...............................................................................9 6.3 Transformation .........................................................................................10 6.4 Geoid Model.............................................................................................11 6.5 Survey Control Co-ordinates in the National System ...............................11 6.6 Costs ........................................................................................................12

7. The Survey Regulations...............................................................................12 8. Recommendations.......................................................................................12

8.1 Mapping Datum........................................................................................12 8.2 Realisation ...............................................................................................12 8.3 GPS Datum..............................................................................................12 8.4 Active Layer .............................................................................................13 8.5 Passive Layer...........................................................................................13 8.6 Realisation of National GPS Network.......................................................13 8.7 Transformation .........................................................................................13 8.8 Gravity......................................................................................................13 8.9 Work on Geoid Model...............................................................................13 8.10 Adoption of Geoid Model ......................................................................13

9. Acknowledgements......................................................................................13 10. References ..................................................................................................14 Annex A – Individuals met during research.........................................................16 Annex B – Invitation to Meeting ..........................................................................17 Annex C – Attendees of the Geodesy Meeting ...................................................18



Annex D - The Active Layer ................................................................................19 Annex E - Estimated Costs .................................................................................20

1. INTRODUCTION This report has been prepared by Geodesy consultant, Ian Wilson of Ordnance Survey. The report details the findings of the Geodesy consultant and gives the background and basis for the recommendations made in the geodesy category of land surveying. Indicative costs for each recommendation have been prepared.

2. SCOPE The project also has consultants in both hydrography (Richard Wylde) and engineering surveying (David Powell). These consultants have agreed that the scope of the geodetic work will encompass :-

a) the geodetic datums used onshore b) transformations between the various datums (onshore) c) Geoid Model d) the supply of national control into survey sites

Although this scope has been agreed, the consultants are working together to ensure that recommendations are not contradictory.

3. CURRENT SITUATION IN TRINIDAD AND TOBAGO

3.1 Research

Meetings have been held with individuals who have an interest in geodesy and the use of GPS in and around Trinidad and Tobago. A full list is at Annex A. Several papers relating to the situation in Trinidad and Tobago have been obtained. Reports on work completed at various times in the last century were located and have been used as references. In addition to the above meetings with individuals a group meeting was arranged for all professionals in geodesy. An advertisement was placed in the Trinidad and Tobago Guardian on Saturday 29 January 2000. A copy is at Annex B – Invitation to Meeting and a list of attendees is at Annex C – Attendees of the Geodesy Meeting.

3.2 Datums in use

The first geodetic datum for Trinidad and Tobago was established in the early part of the last century. This first datum is usually known as ‘the Cassini’ (although this describes the projection, rather than the datum).



Further work was carried out in the mid 20th century which resulted in the ‘Naparima 1955’ datum. More complete details of these and other datums is contained in Wylde, 2000 and Philip & Ramon-Fortuné, 1975. The existing geodetic networks have not been maintained for some time. This is leading to a loss of marks which, eventually, will make the networks useless. There is some evidence that this effect is already being felt. The situation with the vertical datum is less clear. It is known that a primary loop has been completed around Trinidad and that this misclosed by ‘about a foot’. The only name found for the datum is TGR (Trinidad Government Railway). This datum was realised by assigning a ‘known’ value to the FBM on the station of TGR. It is related to MSL (mean sea level) and it is known that tide gauge bench marks were established in the port area. However the records for this work could not be located. Philip & Ramon-Fortuné say:

“Though both islands have numerous circuits of vertical control points, and though the vertical control is good enough for the largest scale mapping, these circuits do not merit the title ‘geodetic’.”

The situation with datums is exacerbated by the lack of an acceptable geodetic connection between Trinidad and Tobago. Although work on this has been done (Ramon-Fortuné, 1973 and Wilcox (no reference traced) it has not been formally incorporated in the realisation of the Naparima 1955 datum. In this context ‘realisation’ is the process of establishing survey marks on the ground which have known co-ordinates in the chosen datum.

3.3 Current Practice

Where possible survey measurements are now based on the Naparima 1955 datum. Due to station destruction, points based on the Naparima 1955 datum are difficult to find and may be well away from the area of interest. Accordingly surveyors have adopted a pragmatic approach and often use old Cassini stations. This is achieved by applying a '‘transformation' to the Cassini point to obtain co-ordinates in Naparima 1955 – and vice versa. Unfortunately the transformation is always calculated on a local basis. This is undesirable as the values obtained are entirely dependent on the points used. Hence different surveyors using different points will obtain different answers. This leads to disputes regarding the correctness of the surveys. Most cadastral surveys are not related to any datum. Although this approach deals with the short term need to register surveys it has the significant drawback that surveys cannot be related reliably to each other. Again this leads to disputes and significant additional survey effort to resolve such disputes. Philip, 1973 makes clear the defects of this approach. Some surveyors are using the GPS (Global Positioning System). Lands & Surveys have three receivers, some are being used by private surveyors (exact numbers have not been determined) and UWI (University of the West Indies – St Augustine Campus) has two. In the absence of a National GPS Network, the use of GPS is problematical. Current practice is to use the existing geodetic control



(Naparima 1955) and, in effect, compute a local transformation. This involves ‘distorting’ the GPS observations to fit the existing control. As the GPS is intrinsically more precise than the existing control this has the effect of degrading the final answer. A more serious problem is that the answer depends (again) entirely on the particular selection of points used. Hence different surveys obtain different results - giving another source of dispute. While talking to the surveyors it was apparent that some do not trust GPS to give good results.

3.4 Work on a new Geodetic Infrastructure

Saleh and Edwards, 1999 details a proposal for work on existing GPS observations to produce a new National GPS Network for Trinidad and Tobago. These existing GPS observations were principally made by Lands and Surveys Division staff in the mid 1990’s. This proposal was accepted by the Director of Surveys and led to two additional papers viz. Geodetic Services, September 1999 and Geodetic Services October 1999. Both of these papers are expertly written and deal comprehensively with the issues. Geodetic Services, September 1999 gives a comprehensive account of the detailed work undertaken to re-compute the GPS observations. The results could (and should) form the basis of a new National GPS Network for Trinidad and Tobago. This work also classifies the achieved standard of the National GPS Network based on FGCS 1988. Although other standards exist (e.g. LINZ, 1998) those used in Geodetic Services, 1999 are considered the most appropriate for Trinidad and Tobago. Accordingly they have been used in preparing the draft Regulations. Geodetic Services, October 1999 gives an account of the computation of various transformation parameters. The computation is rigorous and the statistical analysis very comprehensive. Again this work could provide a basis for accepted National transformation parameters. The issue of transformation and Geoid models is complex and is dealt with below in section 4.

3.5 Comment

The current practice of using both Naparima 55 and Cassini datums, and using piecemeal transformations between the two is leading to dispute and confusion in the surveys. Even two errorless surveys, based on different realisations of the same datum, will lead to conflicting results. Surveys based on differing datums, or a poorly defined datum, are often impossible to relate to one another. The GPS equipment available in Trinidad and Tobago (mainly Trimble 4000 series receivers) is known to give excellent results. Hence the poor results sometimes obtained may be attributed to the lack of a well defined geodetic datum. This in turn is leading to resistance to the use of GPS (however, the cost of equipment is probably a bigger factor). Two private surveyors interviewed, Mr Winston Mohammed and Mr Sasha Ian Addo, have developed an efficient method of working with GPS. They have achieved this by adopting ‘known’ co-ordinates for some stations and using the



same transformation throughout Trinidad and Tobago. This is exactly the thrust of the recommendations made in this paper, but with appropriate attention paid to national standards. However the surveyors are using different parameters and, what’s more, they differ from those given in Geodetic Services, October 1999. The various parameters are shown in the table below.

Geodetic Services Surveyor A Surveyor B DMA1 Diff X -0.216 -0.460 -0.528 -2

Diff Y 372.252 372.397 372.636 374

Diff Z 172.231 172.295 172.345 172

NWG, 1998 gives a detailed account of the aerial triangulation undertaken to produce data for Digital Mapping produced from the 1994 photography. In this case a “two-dimensional transformation routine” was used to transform between WGS84 and Naparima datum. The exact version of Naparima used (1955 or 1972) is not stated and the WGS84 co-ordinates were based on a single station at the airport. This work presents another example of contractors producing a local solution because an accepted, clearly defined and accessible, national solution does not exist. It is probable that other parameter sets and solutions exist. The existing geodetic network in Trinidad and Tobago is no longer adequate for modern needs.

4. THE NEED FOR A GEODETIC DATUM

4.1 What is a Geodetic Datum?

Any attempt to represent the features on the earth in some model (map) requires the definition of a co-ordinate system that is unambiguous and unique. Such a definition is known as a geodetic datum. The primary purpose of the geodetic datum is to provide a referencing system to which positions of features may be related. In the past the area over which a datum could be used was limited and so many such datums exist. Because co-ordinates for the same point have different co-ordinates in each datum it is essential that the datum used is known and well defined. Unfortunately this is not the case in Trinidad and Tobago. The situation is further complicated by the fact that no reliable connection exists in either datum between the two islands of Trinidad and Tobago. This omission is due to the difficulties in establishing such a connection using ‘conventional’ surveying equipment. It seems beyond doubt that the most used datum is Naparima 1955 consequently there exists a significant amount of work in this

1 The Defense Mapping Agency of the USA. TR8350.2 December 1987.



datum. It is recommended that this datum continues to be used for land survey in Trinidad and Tobago.

4.2 Why is a datum needed?

Surveys are carried out for many purposes. Maps and plans are prepared in many disciplines such as land management, cadastre, resource exploitation, nature conservation etc. If these diverse data sources are to be compatible then they must all use a common co-ordinate system. For example, a conservationist may map an area of special interest which the Authorities seek to have protected; later an engineer designs a road which passes through the same area. Unless both specialists base their measurements on the same co-ordinate system the road engineer will be unable to relate the position of the road to the position of the nature conservation area. Another example could involve two nations who seek to define their boundary in an unambiguous way. An internationally accepted datum will allow this work to proceed and the results to be widely accepted. A well defined, and accessible, geodetic datum will allow all surveys to relate to each other. This then allows information to be exchanged and used with maximum benefit to all concerned. The need for a coherent co-ordinate system is not new. Philip, 1973 gives an excellent account of why a national co-ordinate system is so important. Although written nearly thirty years ago the justification presented is still valid today and is well worth re-visiting.

4.3 The Effect of GPS

GPS is becoming the positioning tool of choice. However its use in surveying requires the use of new techniques if reliable, and accurate, results are to be obtained. Because GPS is ‘so easy’ and the price of receivers is falling, many disparate disciplines are taking up GPS when they require the determination of position. Examples are –

a) Conservation b) Resource exploitation c) Forestry management d) Agriculture e) Education f) Security (Police & Defence) g) Port management h) Fishery management i) Boundary demarcation



Such a wide use of GPS inevitably leads to disputes between the practitioners about ‘who is right’. Without a well defined geodetic datum, with corresponding transformation and Geoid model, it is impossible to say ‘who is right’. These ambiguous positions can lead to extremely serious disputes (e.g. uncertainty over International boundaries). This wide use of GPS, entirely separate from the surveying profession, makes the establishment of an accepted and accessible geodetic datum very important. The establishment of such infrastructure is rightly in the province of the geodesist and land surveyor. Another important aspect of GPS is its reliance on WGS84 datum. All measurements with GPS are related to this datum. However the co-ordinate of choice in Trinidad and Tobago is Naparima 1955. Hence to use GPS effectively it is also necessary to define the relationship between these two datums. Such a definition is known as a ‘geodetic transformation’. A similar consideration exists for vertical positions; here the relationship between the two datums is known as a ‘Geoid Model’.

4.4 Accessibility

Easy access to the datum is vital otherwise it will not be used. In the past a geodetic datum was made accessible by building monuments at regular intervals, usually on hill tops (known as ‘the realisation’). These monuments were expensive to install, and remain very expensive to maintain. Unless regular maintenance is undertaken the monuments decay and the ‘accessibility’ of the datum is compromised. In the absence of such monuments surveyors have no economic means of relating their work to the datum. Hence piecemeal realisations proliferate.

5. GEODETIC ‘TOOLS’

5.1 Transformations

The need for transformations is entirely brought about by the use of multiple datums. In this case we need to refer GPS derived positions to those in the local datum (Naparima 1955). Conversely to use GPS with the local datum it is essential we have an accepted transformation. It is clear that all practitioners should use the same transformation. Less obvious is the fact that all practitioners must also use the same realisation of each datum. Here we are primarily concerned with the use of ‘GPS co-ordinates’. It is essential that such co-ordinates are well determined in the GPS datum.

5.2 Geoid Model

The need for a Geoid Model is brought about by the use of GPS. Heights determined by GPS are related to the GPS datum which is different to the local vertical datum (TGR – Trinidad Government Railway). A Geoid Model allows results from GPS to be converted to results in the local datum.



Again it is absolutely essential that a single model is used and that answers in the GPS datum are related to some defined realisation of that datum.

6. THE FUTURE

6.1 The Datum Issue

The de facto mapping datum in Trinidad and Tobago is Naparima 1955. Some work has been done on a re-computation of the observations for this network. Such work would produce a homogeneous set of co-ordinates for Naparima 1955. However, unless such new co-ordinates were then used to re-compute or re-cast all existing work their existence would be of no practical significance. As it would be very costly and also extremely difficult to carry out such work it is recommended that the existing co-ordinates are adopted as the official realisation of Naparima 1955. This should be reflected in the Survey Regulations. This network should be known as the National Triangulation Network. The achieved accuracy of this network is indirectly analysed in Geodetic Services, October 1999. This paper will provide a basis for setting quality standards in the Regulations. For the avoidance of doubt, it should be made clear that this adoption should not extend to co-ordinates which are known, or can be shown to be (to the satisfaction of the Director of Surveys), in error. Where such is the case the co-ordinate should be re-determined or the point destroyed. There is no accepted datum for GPS measurements. The most commonly used international datum is ITRF96 (International Terrestrial Reference Frame 1996). Nearby countries have adopted this datum for their new networks (MSI, 1996). IERS, 2000 now gives a definition for ITRF2000. It is recommended that an ITRF, is used for the National GPS Network. The precise year and epoch of ITRF should be decided at computation stage.

6.2 National GPS Network

Work on this has already begun and is detailed in Geodetic Services, September 1999. This forms a sound basis for future work and should be used to begin the realisation of a new National GPS Network. The paper (Geodetic Services, September 1999) is very useful in defining accuracy standards for any new work. The achieved standard of the existing GPS work is assessed and will provide a useful basis for setting standards in the Regulations. The key aspect of any new network is accessibility. In many countries of the World this is being achieved by the use of permanent GPS stations. Such stations are often referred to as a CORS (Continually Operating Reference Station) network. These can be thought of as the ‘Active Layer’ of the National GPS Network while the more usual monuments (bolts, pillars etc) are termed the ‘Passive Layer’. The observations from the Active Layer are made available, usually via a Web Site, to all practitioners who may require them. This approach has significant benefits.



a) All users have access to the same co-ordinate system. b) Access is easily achieved via the Internet. c) Users require less GPS receivers as stations in the National GPS

Network do not need occupying. d) Users of GPS need not visit remote triangulation stations and so

the cost of surveys is reduced. e) Surveys can be completed more rapidly and with greater

confidence in the results (e.g. rapid survey of land parcels in both the agricultural and urban sectors).

f) the need for a dense network of monuments is reduced, hence maintenance costs are reduced.

It is recommended that either 4 or 5 such stations are established on Trinidad and Tobago. The exact distribution of those stations needs more research. The Active Layer should be administered, and maintained, by the Lands and Surveys Division. Once established the Active Layer could provide significant benefits to positions established offshore, see Annex D - The Active Layer. Once established it is of critical importance that the Active Layer is maintained. The Active Layer should be augmented by the Passive Layer. This is the existing stations whose positions have been determined in Geodetic Services, September 1999. The positions and locations of these stations should be stored in an electronic database which is also available on the Internet. It is recommended that a database of Passive Stations is compiled. It is clear that the Active and Passive Layers must be determined on the same datum and in the same realisation. This may be achieved by observing connections between the two layers and performing a homogeneous solution of all stations (i.e. the whole National GPS Network). It is recommended that connections between the Active and Passive Layers are observed. It is further recommended that a single computation of the whole National GPS Network is completed. The results obtained will form the realisation of the National GPS Network for Trinidad and Tobago.

6.3 Transformation

This area needs more research. If the transformation is to be useful then it must enable the conversion of co-ordinates between the two datums at a suitable accuracy (say 0.200m). Although tests indicate that this has been achieved when comparisons are made with geodetic marks, it may not be the case when compared to lower order survey control points (on which existing surveys are based). If the Active Layer is available the collection of additional test points for the transformation is greatly simplified. Only one surveyor, complete with geodetic



grade GPS receiver (Lands & Surveys Division already owns three) is required to collect the data. Moreover between 4 and 8 points could be completed in a day depending on travel required. Of particular interest in this area is the densification of the existing National GPS Network. It is possible to use existing non-GPS observations to densify the network. This method has the considerable advantage of being relatively inexpensive compared to any approach that requires additional GPS observations. It is recommended that research is undertaken to determine a practical transformation for Trinidad and Tobago. This should specifically address the errors which may exist in the lower order control.

6.4 Geoid Model

No Geoid Model of sufficient accuracy exists for Trinidad and Tobago. Work has already been suggested (Saleh & Edwards, 1999) that would address this problem. It is certain that more gravity data is required to enable an acceptable model to be computed. It is recommended that either a gravimeter is purchased, hired or a contract arranged, to secure additional gravity data. This applies especially in the Northern Range of Trinidad. It is further recommended that the current work being undertaken by Geodetic Services is completed. This will secure a usable Geoid Model for Trinidad and Tobago. It is recommended that after suitable testing the new model is adopted as the official Geoid Model for Trinidad and Tobago. The observations and computations involved in producing a reliable Geoid Model is a specialist area of Geodesy. Very few experts exist who are able to do this work with confidence. If required Ordnance Survey could recommend suitable contractors who may be willing to undertake this work.

6.5 Survey Control Co-ordinates in the National System

Of prime interest to any surveyor is the provision of control co-ordinates in the national system. This is usually the means by which a cadastral or topographic surveyor gains access to the geodetic datum. The existence of an Active Layer and the necessary geodetic tools, greatly simplifies the provision of national control with GPS. Assuming average distances from Active station to point of interest of about 60 kilometres a new station could be accurately co-ordinated (about ± 0.050m) with about 30 minutes of observations. This can be done anywhere in Trinidad and Tobago where a clear view of the sky is available. Once established these control points can be used with Total Stations (or theodolites) to survey the plans or maps. Where access to GPS is not available surveyors would need to use the Passive Layer. This will be less efficient than GPS with existing densities of trig control.



However the density of the control network can be enhanced by using GPS and this work could be done very efficiently by the Lands & Surveys Division with existing equipment.

6.6 Costs

Where possible actual costs have been obtained. These are mainly restricted to hardware costs. To provide an indication of manpower costs estimates of time and skill level have been given. These estimates are given in Annex E - Estimated Costs.

7. THE SURVEY REGULATIONS To derive maximum benefit from the surveys carried out in Trinidad and Tobago it is essential that all surveys refer to the same realisation of the same datum. This should be specifically included in the survey regulations. If surveys in and around Trinidad and Tobago are to use GPS effectively it is essential that the following are defined in the Regulations:-

a) The datum to be used for surveying b) The datum to be used for GPS c) The transformation between the two datums d) The Geoid model to be used with GPS

Standards are also appropriate for the Regulations. However at this stage it is impossible to provide reasonable estimates of the standards that should be achievable in Trinidad and Tobago. Consequently it is undesirable to provide standards for the Regulations as this may incur significant expense in achieving them. The draft regulations have been written in such a way that the standard can be established by the Director of Surveys defining two parameters to be used in assessing achieved accuracy. Reference should be made to both the papers by Saleh and Edwards. This is also the approach taken in FGCS, 1988.

8. RECOMMENDATIONS It is recommended that -

8.1 Mapping Datum

The Naparima 1955 datum should continue to be used for land survey in Trinidad and Tobago (4.1)

8.2 Realisation

The existing co-ordinates should be adopted as the official realisation of Naparima 1955 (6.1).

8.3 GPS Datum

An ITRF based datum should be used for a new National GPS Network (6.1).



8.4 Active Layer

Either 4 or 5 GPS base stations should be established on Trinidad and Tobago (6.2).

8.5 Passive Layer

A database of Passive Stations should be compiled and made available to all interested parties (6.2).

8.6 Realisation of National GPS Network

A single computation of the whole National GPS Network should be completed (6.2).

8.7 Transformation

Research should be undertaken to determine a practical transformation for Trinidad and Tobago. Such a transformation should then be adopted in the Survey Regulations (6.3).

8.8 Gravity

Either a gravimeter should be purchased, hired or a contract arranged, to secure additional gravity data (6.4).

8.9 Work on Geoid Model

The current work to produce a new Geoid Model, being undertaken by Geodetic Services, should be completed (6.4).

8.10 Adoption of Geoid Model

After suitable testing the new model should be adopted as the official Geoid Model for Trinidad and Tobago (6.4).

9. ACKNOWLEDGEMENTS Thanks are due to all the surveyors who helped me compile this report. In particular I would like to thank Mr Anthony Ramon-Fortuné for kindly lending me copies of some important reports. Thanks are also due to Ms Karla Edwards whose patience in explaining the situation regarding geodesy on Trinidad and Tobago is very much appreciated. Last, and by no means least, I would like to thank the Director of Surveys Mr Tyrone Leong for his co-operation while I was researching material for this report.



10. REFERENCES

Wylde, January 2000 Marine Geodesy as applied to Oil and Gas exploration and production Offshore Trinidad and Tobago. Note from Richard Wylde to Fred Brazier dated 24th January 2000.

Saleh and Edwards, 1999 Building the Geodetic Infrastructure of Trinidad and Tobago in the GPS Satellite era. A Proposal prepared by Jarir Saleh and Karla Edwards for The Lands and Surveys Division of Trinidad and Tobago dated 23rd June 1999.

Geodetic Services, September 1999 Adjustment and Analysis of the GPS Network of Trinidad and Tobago. A provisional study paper, only available from the Director of Surveys, prepared by Geodetic Services, dated 3rd September 1999.

Geodetic Services, October 1999 The Computation of Accurate Transformation Parameters for Trinidad and Tobago A provisional study paper, only available from the Director of Surveys, prepared by Geodetic Services, dated 27th October 1999.

Ramon-Fortuné, 1973 A Geodetic Connection between Trinidad and Tobago Paper presented to the Conference of Land Surveyors of the Trinidad and Tobago Public Service 1973. Copy held by Mr Arnold J Ramon-Fortuné and kindly made available for this report.

Philip, 1973 Where do we go from here? Paper presented to the Conference of Land Surveyors of the Trinidad and Tobago Public Service 1973.

Copy held by Mr Arnold J Ramon-Fortuné and kindly made available for this report.

Philip & Fortuné, 1975 Geodetic and Topographic Surveys in Trinidad and Tobago Paper presented to the Conference of Land Surveyors of the Trinidad and Tobago Public Service 1975. Copy held by Mr Arnold J Ramon-Fortuné and kindly made available for this report.

MSI, 1996. (Measurement Science Inc) Datum Determination, Geodetic Control and GPS Base Stations in Belize. July 1996 Final Report , Volume 2.



FGCS, 1988 Geometric geodetic standards and specifications for using GPS relative positioning techniques. Version 5.0. Federal Geodetic Sub-committee, National Geodetic Survey, National Geodetic Information Branch. Silver Spring, Maryland, USA.

LINZ, 1998. Accuracy Standards for Geodetic Surveys. OSG Standard 1. Land Information New Zealand. Office of the Surveyor General.

IERS, 2000 World Wide Web site http://hpiers.obspm.fr/ Web site of the International Earth Rotation Service

NWG, 1998 Trinidad and Tobago Aerial Triangulation & Adjustment Project. Preliminary Report. Report produced by North West Geomatics Ltd. Alberta, Canada. ([email protected]). January 14, 1998.

http://hpiers.obspm.fr/

mailto:[email protected])



ANNEX A – INDIVIDUALS MET DURING RESEARCH. Tyrone Leong, Director of Surveys Dr Thackwray Driver Senior Implementation Officer Alan Williams LUPAP Project Manager Dr A T Philip Registrar to Land Survey Board Karla Edwards M Phil Student, UWI Andrew Bowles Surveyor 3, Lands & Surveys Michael Jones Surveyor 3, Lands & Surveys Winston R Mohammed Licensed Land Surveyor Arnold Ramon-Fortuné Licensed Land Surveyor Michael Paul Topographer, Lands & Surveys Elliot McKell Topographer, Lands & Surveys Ramesh Badri Marketing Rep, Western Scientific Dr Keith Miller Lecturer in Geodesy, UWI Dr Raid Al-Tahir Lecturer in Photogrammetry, UWI Richard Cattermole Hydrographer, Cane Associates David Neale Hydrographer, Cane Associates Sasha Ian Addo Licensed Land Surveyor



ANNEX B – INVITATION TO MEETING

Ministry of Agriculture, Land and Marine Resources and Ministry of Housing and Settlement

NOTICE

Information Circular to Surveyors of Trinidad and Tobago LAND SURVEYORS REGULATIONS: GEODESY CATEGORY

Land Use Policy and Administration Project: LUPAP

Surveyors and persons interested in geodesy are invited to attend a:

Meeting to be held at 4:30 p.m. on Wednesday 2 February 2000 at the Lands and Surveys Division, 118 Frederick Street, POS.

Ordnance Survey’s Ian Wilson, who has an MSc in geodesy and has been working with GPS since 1988, will make a presentation on:

• The need for geodetic datum

• Developments world-wide – how GPS is changing geodesy

• The situation in Trinidad and Tobago:

• Establishing the datum

• Provision of control for surveys

• The need for an operational framework - regulations or best practice guidelines

• Future developments

Your attendance at this Meeting would be very much appreciated to ensure that we are aware of the views of surveyors who are interested in geodesy and GPS. For further information contact:

Fred Brazier 633 8948 or 625 0427 email: [email protected]

Charisse Griffith-Charles 640 2959 or 662 2002 Ext. 3314 email: [email protected]

mailto:[email protected]

mailto:[email protected]



ANNEX C – ATTENDEES OF THE GEODESY MEETING

No. Name Speciality / Profession 1 Keith Miller Geodesy, Hydrography Lecturer

2 Carl Williams Geodesy

3 Cuthbert Brazil Geodesy

4 Roland Wiseman Surveyor

5 Winston Ramcharan Surveyor

6 Alicia Naimool Surveyor

7 Tyrone Leong Director of Surveys

8 Glenn Wilkes Surveyor

9 Richard Cattermole Hydrographic Surveyor

10 Karla Edwards Geodesy

11 Stephanie Elder-Alexander Surveyor

12 Ramesh Badri Surveyor

13 Arnold Ramon-Fortuné Surveyor

14 Marion Mohammed Surveyor

15 Winston Mohammed Farmer

16 Clinton R. Stewart Technician

17 Paul Wilson Attorney at Law

18 Bhaghirathi Maharj Surveyor

19 Ainsley Charles UWI (Student)

20 Roopchand Deonanan Surveyor

21 Shri Persad Maharaj Surveyor

22 M Chin Chow Surveyor



ANNEX D - THE ACTIVE LAYER While the establishment of the Active Layer has significant benefits for land surveying and positioning on land, it also could have benefits for offshore users. The Active stations provide an ideal resource for the use of marine navigators. IALA (International Association of Lighthouse Authorities) has established GPS beacons around many coastlines. Significantly the United States of America is extending this service to all land areas. The beacons consist of radio transmitters (medium wave band) which broadcast corrections to the GPS signal. These can be received, free of charge, by receivers suitably equipped. These navigation receivers are generally low cost (circa $TT 2,000). One (or possibly two) stations in the Active Layer could be augmented by transmitters to make these signals available to all navigators. This would allow positions to be determined, in real time, to an accuracy of a few metres. This would greatly relieve the problems surrounding the exact position of the international boundary and so help in the resolution of disputes concerning fishing and mineral rights etc. Experience in Great Britain indicates that the signals from these beacon stations (GB has 8) can be used by Land Surveyors. Recent experiments have shown the positions derived form such beacons to be accurate to about ±1.0m. Such accuracy can be useful in topographic mapping applications. It should be remembered that this is achieved with comparatively low cost GPS survey equipment (circa $TT 40,000) and only one such receiver is required. This possibility may be more fully explored in the Hydrography Report.



ANNEX E - ESTIMATED COSTS Costs are calculated for each recommendation. The costs given are indicative only. Mapping Datum

The Naparima 1955 datum should continue to be used for land survey in Trinidad and Tobago ( This recommendation is made specifically to reduce costs. Using any other datum would involve considerable cost in converting existing data. Cost - nil.

Realisation The existing co-ordinates should be adopted as the official realisation of Naparima 1955 (6.1). This recommendation Is made with a view to reducing costs. Adopting the existing co-ordinates involves no cost. Cost – nil.

GPS Datum An ITRF based datum should be used for a new National GPS Network (6.1). This can be achieved by using suitable computational techniques when determining the National GPS Network. Cost will be negligible and limited to obtaining the observations from international control stations (via the Internet). Cost 3 days Geodetic Expert

Active Layer Either 4 or 5 GPS base stations should be established on Trinidad and Tobago (6.2). This is a significant investment. The majority of this work can be completed from resources available in Trinidad and Tobago. Consultancy costs would be restricted to about one week (Geodesy & GPS expert).

a) Base Station Hardware 5 stations @170,000 $TT 850,000 b) Auxiliaries (PC’s etc) 5 sets @ 20,000 $TT 100,000 c) GPS Server (incl. software)1 only $TT 20,000 d) Installation One month Local Land Surveyor e) Installation One week Base Station Expert f) Installation One week Network Expert

Cost $TT 970,00 plus Manpower



Passive Layer A database of Passive Stations should be compiled and made available to all interested parties (6.2). It is assumed that all information will be available in Lands & Surveys Division.

a) Design 5 days Geodesy Expert b) Data Entry One month Land Surveyor / Data processor c) Access (Internet) Two days Network Expert d) Materials assumes computer available (GPS server)

Realisation of National GPS Network A single computation of the whole National GPS Network should be completed (6.2). This work would be a by-product of Recommendation 8.3. Cost – nil.

Transformation Research should be undertaken to determine a practical transformation for Trinidad and Tobago. The costs given here are dependant on Recommendation 8.4 being implemented. In the absence of the Active Layer the data gathering costs could escalate by 150%.

a) Data gathering two months Land Surveyor b) Data processing one week Geodetic Expert c) Report writing two days Geodetic Expert

Gravity Either a gravimeter should be purchased, hired or a contract arranged, to secure additional gravity data (6.4). Current cost of a Gravimeter is

Work on Geoid Model The current work to produce a new Geoid Model, being undertaken by Geodetic Services, should be completed (6.4).

a) Research two months Geodetic Expert b) Report writing one week Geodetic Expert Note that the accurate costs could be obtained by quotation from Geodetic Services.

Adoption of Geoid Model After suitable testing the new model should be adopted as the official Geoid Model for Trinidad and Tobago (6.4). Cost - nil.

geodesy category report - terra institute category report prepared by ian wilson ... st augustine...

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