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APJ ABDUL KALAM TECHNOLOGICAL UNIVERSITY
B.Tech DEGREE MODEL EXAMINATION NOVEMBER 2017
CE 307 GEOMATICS
Time : 3 hrs Max. Marks : 100
ANSWER KEY
PART A
1. The term balancing is generally applied to the operation of applying corrections to
latitudes and departures so that ∑L = 0 and ∑D = 0. This applies only when the
survey forms a closed polygon. The following are common methods of adjusting
traverse:
i) Bowditch’s method
ii) Transit method
iii) Graphical method
iv) Axis method
i) Bowditch rule: The basis of this method is on the assumptions that the errors in
linear measurements are proportional to l where l is the length of a line. The
Bowditch’s rule is also termed as the compass rule. It is mostly used to balance a
traverse where linear and angular measurements are equal precision. The total
error in latitude and in the departure is distributed in proportion to the lengths of
the sides. Bowditch Rule is Correction to latitude (or departure) of any side =
Total error in latitude (or departure) X perimeteroftraverse Lengthofthatside
Thus if CL = Correction to latitude of any side
CD = Correction to departure of any side
∑L = total error in latitude
∑D= total error in departure
∑ l = length of the perimeter
l = length of any side
We have CL = ∑L x l l and CD = ∑D x l l
ii) Transit Method: The transit rule may be employed where angular measurements
are more precise that the linear measurements. According to this rule, the total
error in latitudes and departures is distributed in proportion to the latitudes and
departures of the sides. It is claimed that the angles are less affected by
corrections applied by transit method that by those by Bowditch’s method.
The transit rule is: Correction to latitude (or departure) of any side = Total error in
latitude (or departure) X ( ) ( ) Arithmeticsumoflatitudes departures Latitude departure of
that line
Where L = latitude of any line
D = departure of any line
LT = arithmetic sum of latitudes
DT = arithmetic sum of departure We have,
CL = ∑L x LT L and CD = ∑D x DT D
iii) Graphical method: For rough survey, such as a compass traverse, the Bowditch
rule may be applied graphically without doing theoretical calculations. According
to the graphical method, it is not necessary to calculate latitudes and departures
etc. however before plotting the traverse directly from the field notes, the angles
or bearings may be adjusted to satisfy the geometric conditions of the traverse.
The polygon AB’C’D’E’A’ represents an unbalanced traverse having a closing error
equal to A’A since the first point A and the last point A’ are not coinciding. The total
closing error AA’ is distributed linearly to all the sides in proportion to their length by a
graphical construction shown in figure. AB’, B’C’, C’D’ etc are represent the length of
the sides of the traverse either to the same scale or reduced scale. The ordinate aA’ is
made equal to the closing error A’A. by constructing similar triangles, the corresponding
errors bB’, cC’, dD’, eE’ are found. The lines eE’, dD’, cC’, bB’ respectively. The
polygon ABCDE so obtained represents the adjusted traverse. It should be remembered
that the ordinates bB’, cC’, dD’, eE’, aA’ represent the corresponding errors in magnitude
only but not in direction.
iv) Axis Method: This method is adopted when the angles are measured very
accurately, the corrections being applied to lengths only. Thus the only directions
of the line are unchanged and the general shape of the diagram is preserved. To
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adjust the closing error aa1 of a traverse abcdefa1 the following procedure is
adopted
Join a1a and produce it to cut the side cd in x.
the line a1x is known as the axis of adjustment.
The axis divides the traverse in two parts i.e. a b c x and a1 f e d x. Bisect a1a in
A.
Join xb, xe and xf. Through A, draw a line AB parallel to ab cutting xb produced
in B.
through B, draw a line BC parallel to bc cutting xc produced in C.
Similarly, through A, draw AF parallel to a1f to cut xf in F. through F, draw FE
parallel to fe to cut xe in E. through E, draw ED parallel to ed to cut xd in D.
ABCDEF (thick lines) is the adjusted traverse.
The axis a1x should be so chosen that it divides the figure approximately into two equal
parts. In some cases the closing error aa1 may not cut the traverse or may cut it in very
unequal parts. In such cases, the closing error is transferred to some other point. In figure
aa1 when produced does not cut the traverse in two parts. Through a, a line ae’ is drawn
parallel and equal to a1e. through e’, a line e’d’ is drawn parallel and equal to ed. A new
unadjusted traverse dcbae’d’ is thus obtained in which the closing error dd’ cuts the
opposite side in x, thus dividing the traverse in two approximately equal parts. The
adjustment is made with reference to the axis dx. The figure ABCDE shown by thick
lines represents the adjusted figure.
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3. Instrumental Method
Rankine’s Method.
Two theodolite method .
Tacheometric method
Rankine’s Method.
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Transite theodolite and chain or tape are used in this method. It is normally used for
setting out the curves in railways and other important work.
Formulae for the deflection angle method.
R be the radius of the curve and Ө be the deflection angle.
AB is the first tangent to the curve .
E,D,T1 etc are the successive point on the curve.
δ1, δ2, δ3, δ4, etc are tangential angle or deflection angle made by long chord
T1E,ED,DT2 w.r.t .tangent.
∆1, ∆2, ∆3,etc are the total tangential angle or deflection angle between
i) first tangent AB and
ii) first tangent AB and T1D
iii) first tangent AB and T1T2
C1,C2,C3 etc are the length of chord T1E,ED, DT2
chord T1E =Arc T1E assumed to be C1.
BT1E = δ1 = ½ T1 OE
T1 OE = 2 δ1
Now T1 O E / C1 = 180 / ∏R
T1 O E = (180 / ∏R) X C1
2 δ1 = (180 / ∏R) X C1
δ1 = (180 / 2 x ∏R) X C1
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=( 90 / ∏R ) X C1
= (90 X 60/ ∏R ) X C1 ( 60 minute)
= (5400/∏R ) X C1 ( 60 minute)
δ1 = (1718.9/R) X C1 ( 60 minute)
Similarly, δ2 = (1718.9/R) X C2
δ3 = (1718.9/R) X C3
δ n = (1718.9/R) X Cn
Total Deflection angle calculation.
Total deflection angle or Tangential angle are ∆1, ∆2, ∆3,etc .
Total Deflection angle for first chord.
T1 E = B T1 E ∆1 = δ1
Total deflection angle ∆2 = δ1 + δ2
∆2 = δ1 + δ2 = ∆1 + δ2
∆3= δ1 + δ2 + δ3 = ∆2 + δ3
∆4 = δ1 + δ2 + δ3 + δ4 = ∆3+ δ4.
Check for deflection angle < BT1T2 = ∆n = Ө/2
Setting out of curve procedure
1) Set transit theodolite over the tangent point “T1” centering and leveling.
2) Direct the telescope to A so as to bisect the ranging rod held at B.
3) Set vernier A to the first deflection angle ∆1 then direct telescope along T1E.
4) Take one end of the chain or tape at T1 & arrows on the chain at distance equal to
length of sub-chord ,swing the chain around T1 and bisect the arrow so as to fix the
position on first point E on the curve.
5) Loose the upper plate set vernier to second deflection ∆2 then direct line of sight
along T1D. 6) In this way establish end point on the curve.
7) Enter the reading in field book.
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Two theodolite method
Knowing point B of intersection and tangent length established two more control point
T1 & T2. so BT1 = BT2 = Tangent length
Set up the theodolite at T1 & another theodolite on T2
Set the reading of each transite theodolite are deflection angle ( ∆1) for the first point ( E
) .the line of sight of the both the theodolite are thus directed toward ( E ) along T1E ,&
T2E resp.
To fix the second point ‘F’ set reading ( ∆2) on the both theodolite.
In order ,locate all other point.
Tacheometric method
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Set the tacheometer at �1 and sight the point of intersection when the reading is zero.
Set the deflection angle ∆1 on the vernier, thus directing the line of sight along �1A.
Direct the staff man to move in the direction �1A till the calculated staff intercept �1 is
obtained. The staff is generally held vertical. First point A is fixed.
Set the deflection angle ∆2 directing the line of sight along �1B.
Move the staff backward or forward untill the staff intercept �2is obtained thus fixing the
point B.
Same other points are fixed.
4. Code measurement
A GPS receiver determines the travel time of a signal from a satellite by
comparing the "pseudo random code" it's generating, with an identical code in the
signal from the satellite. The receiver "slides" its code later and later in time until it
syncs up with the satellite's code. The amount it has to slide the code is equal to the
signal's travel time. The problem is that the bits (or cycles) of the pseudo random
code are so wide they aren't perfectly synced. As a result code measurements are
precise to the meter level.
Carrier phase measurement
The carrier phase measurement is a measure of the range between a satellite and
receiver expressed in units of cycles of the carrier frequency. This measurement can
be made with very high precision (of the order of millimeters), but the whole number
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of cycles between satellite and receiver is not measurable.
A good analogy to this is to imagine a measuring tape extending from the satellite to
the receiver that has numbered markers every one millimeter. Unfortunately,
however, the numbering scheme returns to zero with every wavelength
(approximately 20 centimeters for GPS L1). This allows us to measure the range very
precisely, but with an ambiguity in the number of whole carrier cycles.
The carrier phase measurement is much higher so its pulses are much closer together
and therefore more accurate. The pseudo random code has a bit rate of about 1 MHz
but its carrier frequency has a cycle rate of over a GHz (which is 1000 times faster!)
At the speed of light the 1.57 GHz GPS signal has a wavelength of roughly twenty
centimeters, so the carrier signal can act as a much more accurate reference than the
pseudo random code by itself. Carrier phase measurements are able to provide a
precision at the millimeter level.
5. Components of GPS Working Mechanism Space segment:
Composed of satellites that transmit signals from space, on the basis of which time and position of the user is measured.
Set of satellites is called as constellation. GPS uses two satellite constellations i.e. NAVSTAR and GLONASS. NAVSTAR (Navigation satellite timing and ranging) NAVSTAR composed of 24 satellites, arrayed in 6 orbital planes, inclined 55 degrees to
the equator and with a 12 hours period. They orbit at altitudes of about 20,200km each. Each satellite contains four precise atomic clocks, only one of which is in used at a time.
Control segment:
Control segment consists of a group of 5 ground based monitor stations, three antennas and a master control station.
The Master Control facility is located at Schriever Air Force Base (formerly Falcon AFB) in Colorado.
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The monitor stations measure signals from the SVs continuously and provides data to the master control station.
The master control station calculates satellite ephemeris and clock correction coefficients and forwards them to an antenna.
The antenna transmit the data to each satellite at least once a day. The SVs then send subsets of the orbital ephemeris to GPS receivers over radio signals
User segment:
GPS User Segment consists of the GPS receivers and the user community. The typical receiver is composed of an antenna and preamplifier, radio signal
microprocessor, control and display device, data recording unit, and power supply. GPS receivers convert SV signals into position, velocity, and time estimates. A minimum
of four satellites are required to compute the four dimensions of X, Y, Z (position) and Time.
GPS signal structure Satellites have highly precise oscillators with a fundamental frequency of 10.23 MHz. Satellite signals basically consists of 3 componnts (Figure 6.2):
1. Two micro wave L-band (also called Carrier) waves o L1 carrier: 1575.42 MHz o L2 carrier: 1227.60 MHz
2. Ranging codes modulated on the carrier waves o C/A code, the clear/access or coarse/acquisition code modulated at 1.023 MHz,
degraded code for civilian users, modulated on L1 only o P (Y) code, the private, protected, or precise code modulated at 10.23 MHz. It is
modulated on both L1 and L2 carrier waves, for authorized military users 3. Navigation message
o Modulated on both L1 and L2 and contains satellite positions and constants
Two different frequencies are used to eliminate errors introduced by ionospheric refraction.
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All frequencies derived from the fundamental frequency ( f = 10.23 MHz) with the
following frequencies and wavelengths
L1 = 154 f = 1575.42 MHz, wavelength of 19.0 cm,
L2 = 120 f = 1227.60 MHz, wavelength of 24.4 cm
C/A code = (1/10 f ) = 1.023 MHz (Mega bits per second Mbps), wavelength of 293.1 m, period of 1 millisecond
P code = 10.23 MHz, wavelength of 29.31 m, period of 266 days; 7 days/satellite
Navigation code = 50 bits per second (bps), data signal cycle length of 30 seconds The structure of GPS carrier signals, codes and their combinations is quite complex.
Since the carriers are pure sinusoids, they cannot be used easily for instantaneous positioning purposes and therefore two codes are modulated onto them: the C/A (coarse acquisition) code and P (precise) code. The codes (P and C/A) are nothing but binary sequence of information generated by a complicated algorithm.
6. PLANNING AND PREPARATION
GPS Survey must be carefully planned for easy execution and good results. Since GPS Surveying is different from conventional surveying, it requires different planning, execution and processing techniques. Before deciding whether or not to expend resources on planning, one must decide the ultimate purpose of survey and the accuracy of the desired results.
Before start of the GPS survey, some points must be considered as listed below:
Accuracy must be considered in survey work.
Hardware and software available for GPS survey, if GPS data used by different types of
manufacturer, then supplied software should be able to convert it into a desired format or
compatible format, (RINEX : Receiver Independent Exchange format it is used in
different types of receiver for data interchange).
How many personnel requirement.
Survey control, horizontal, vertical or both.
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How many survey controls required etc.
For large projects with many sites and many receivers, planning a GPS Survey could be aided by the use of computer programs to save time and resources.
Careful planning is, therefore, a critical issue and consists of the following elements:
Project Design
This involves a project layout and network design, and is driven primarily by accuracy and station location/density requirements (defined by the client), productivity/economic consideration and standards and specifications (promoted by geodetic control authority).
Observation Schedule
A number of factors need to be considered like number of GPS Receivers, occupation time per site, number of sites per day, requirements for multiple station occupancy etc.
Instrumentation and Personnel
Instrumentation appropriate for the task, mainly driven by what may be available or what could be hired from outside the organisation. Also, adequately trained personnel are needed to carry out the survey and process the data.
Logistical Considerations
The issues such as transportation (to ensure observation schedule can be adhered), special site requirements (eg. Power availability on-site, inter-visibility) and factors related to network design and observation scheduling, such as receiver deployment pattern, etc.
Reconnaissance
A preliminary reconnaissance survey to the site is most needed in GPS surveying. A good reconnaissance is crucial for successful completion of a GPS Survey. The site reconnaissance should ideally be completed before survey starts, the surveyor should also prepare a site sketch and a brief description on how reach the point. Topography of the site and factors affecting establishment of required points can be sought through reconnaissance survey. Hence the required parameters for the survey such as vehicles, type of instrument to be carried, type survey to be conducted, etc. can be prepared before commencing the survey considering the factors below: a) Satellite availability: satellite selection
b) Satellite visibility: checking on-site obstructions
c) Clearly identifying the ground mark over which GPS antenna is to be set up.
d) Station access: critical for minimising non-productive travel times and unscheduled
delays in reaching site.
e) Site conditions
f) Whether open areas exist around stations
g) Whether a control station or established bench mark falls in an area where special
permission is required for its occupation.
h) Whether proper lighting facilities are available for night observations.
a) Satellite availability and Visibility
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To schedule a GPS Survey, the following factors need to be taken into account.
Satellites are not normally tracked below an elevation of 15-200(from horizon) due to
large atmospheric refraction errors at low elevation angles.
There is a 24 hour observation window for GPS.
At least 30 minutes of coverage with a minimum of 4 satellites is the accepted norm for
standard static GPS surveys. There are periods when more satellites are visible.
The satellite positions in sky are predictable.
A popular representation of satellite availability is the Sky Plot, which is a plot of
satellite tracks on a zenithal projection centred at GPS ground station (Fig A).
Sky plots can be used during reconnaissance to ensure that the receiver can acquire satellite signals from the selected satellites. GPS manufacturers softwares can generate the sky plots for each hour. The obstructions at a site can be plotted on a zenith plot independently of sky plot (Fig B). The basic tools for mapping obstruction during a reconnaissance survey are compass and inclinometers. At each site, the obstructions can be mapped and if there are a significant number of potential signal obstructions, they may be visually compared with the various 1 hour sky plots and the best tracking period with minimum obstructions is selected.
b) Station selection, Access and Marking
With details of station access and point description, the area around site should be studied carefully. Depending on the aim of the project and accuracy requirements, this is an elaborate task which includes:
Investigating provision of On-Site Power
Noting presence of any potential multipath causing structures.
Noting any UHF, TV, radio, microwave or radar transmitters that could affect
a receivers operation.
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Establishing permanent monumentation of station point.
Establishing nearby azimuth marks
Clearing area of possible obstructions caused by trees, shrubs, etc.
Required resources:
Some resources that must be required at time of preparation for GPS surveys are listed below:
- How many personnel
- Receivers with accessories
- Transportation facilities
- Images or Maps
- Satellite and Weather forecast information
- Accommodation
- Availability of equipments.
7. Leaves: A chemical compound in leaves called chlorophyll strongly absorbs radiation in the red and blue wavelengths but reflects green wavelengths. Leaves appear "greenest" to us in the summer, when chlorophyll content is at its maximum. In autumn, there is less chlorophyll in the leaves, so there is less absorption and proportionately more reflection of the red wavelengths, making the leaves appear red or yellow (yellow is a combination of red and green wavelengths). The internal structure of healthy leaves act as excellent diffuse reflectors of near-infrared wavelengths. If our eyes were sensitive to near-infrared, trees would appear extremely bright to us at these wavelengths. In fact, measuring and monitoring the near-IR reflectance is one way that scientists can determine how healthy (or unhealthy) vegetation may be.
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Water: Longer wavelength visible and near infrared radiation is absorbed more by water than shorter visible wavelengths. Thus water typically looks blue or blue-green due to stronger reflectance at these shorter wavelengths, and darker if viewed at red or near infrared wavelengths. If there is suspended sediment present in the upper layers of the water body, then this will allow better reflectivity and a brighter appearance of the water. The apparent colour of the water will show a slight shift to longer wavelengths. Suspended sediment (S) can be easily confused with shallow (but clear) water, since these two phenomena appear very similar. Chlorophyll in algae absorbs more of the blue wavelengths and reflects the green, making the water appear more green in colour when algae is present. The topography of the water surface (rough, smooth, floating materials, etc.) can also lead to complications for water-related interpretation due to potential problems of specular reflection and other influences on colour and brightness. We can see from these examples that, depending on the complex make-up of the target that is being looked at, and the wavelengths of radiation involved, we can observe very different responses to the mechanisms of absorption, transmission, and reflection. By measuring the energy that is reflected (or emitted) by targets on the Earth's surface over a variety of different wavelengths, we can build up a spectral response for that object. By comparing the response patterns of different features we may be able to distinguish between them, where we might not be able to, if we only compared them at one wavelength. For example, water and vegetation may reflect somewhat similarly in the visible wavelengths but are almost always separable in the infrared. Spectral response can be quite variable, even for the same target type, and can also vary with time (e.g. "green-ness" of leaves) and location. Knowing where to "look" spectrally and understanding the factors which influence the spectral response of the features of interest are critical to correctly interpreting the interaction of electromagnetic radiation with the surface.
Bare soil :The surface reflectance from bare soil depends on many factors such as color, moisture content, presence of carbonate and iron oxide content.
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8. Coordinate systems enable geographic datasets to use common locations for integration. A coordinate system is a reference system used to represent the locations of geographic features, imagery, and observations, such as Global Positioning System (GPS) locations, within a common geographic framework.
Each coordinate system is defined by the following:
Its measurement framework, which is either geographic (in which spherical coordinates are measured from the earth's center) or planimetric (in which the earth's coordinates are projected onto a two-dimensional planar surface)
Units of measurement (typically feet or meters for projected coordinate systems or decimal degrees for latitude-longitude)
The definition of the map projection for projected coordinate systems
Other measurement system properties such as a spheroid of reference, a datum, one or more standard parallels, a central meridian, and possible shifts in the x- and y-directions
Several hundred geographic coordinate systems and a few thousand projected coordinate systems are available for use. In addition, you can define a custom coordinate system.
Types of coordinate systems
The following are two common types of coordinate systems used in a geographic information system (GIS):
A global or spherical coordinate system such as latitude-longitude. These are often referred to as geographic coordinate systems.
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A projected coordinate system such as universal transverse Mercator (UTM), Albers Equal Area, or Robinson, all of which (along with numerous other map projection models) provide various mechanisms to project maps of the earth's spherical surface onto a two-dimensional Cartesian coordinate plane. Projected coordinate systems are referred to as map projections.
Coordinate systems (both geographic and projected) provide a framework for defining real-world locations.
A geographic coordinate system (GCS) uses a three-dimensional spherical surface to define locations on the earth. A GCS is often incorrectly called a datum, but a datum is only one part of a GCS. A GCS includes an angular unit of measure, a prime meridian, and a datum (based on a spheroid). The spheroid defines the size and shape of the earth model, while the datum connects the spheroid to the earth's surface. A point is referenced by its longitude and latitude values. Longitude and latitude are angles measured from the earth's center to a point on the earth's surface. The angles often are measured in degrees (or in grads). The following illustration shows the world as a globe with longitude and latitude values: In the spherical system, horizontal lines, or east–west lines, are lines of equal latitude, or parallels. Vertical lines, or north–south lines, are lines of equal longitude, or meridians. These lines encompass the globe and form a gridded network called a graticule. The line of latitude midway between the poles is called the equator. It defines the line of zero latitude. The line of zero longitude is called the prime meridian. For most GCSs, the prime meridian is the longitude that passes through Greenwich, England. The origin of the graticule (0,0) is defined by where the equator and prime meridian intersect. Latitude and longitude values are traditionally measured either in decimal degrees or in degrees, minutes, and seconds (DMS). Latitude values are measured relative to the equator and range from –90° at the south pole to +90° at the north pole. Longitude values are measured relative to the prime meridian. They range from –180° when traveling west to 180° when traveling east. If the prime meridian is at Greenwich, then Australia, which is south of the equator and east of Greenwich, has positive longitude values and negative latitude values. It may be helpful to equate longitude values with x and latitude values with y. Data defined on a geographic coordinate system is displayed as if a degree is a linear unit of measure. This method is basically the same as the Plate Carrée projection. A physical location will usually have different coordinate values in different geographic coordinate systems.
Projected coordinate systems
A projected coordinate system (PCS) is defined on a flat, two-dimensional surface. Unlike a GCS, a PCS has constant lengths, angles, and areas across the two dimensions. A PCS is always based on a GCS that is based on a sphere or spheroid. In addition to the GCS, a PCS includes a
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map projection, a set of projection parameters that customize the map projection for a particular location, and a linear unit of measure.
9. a.The sun provides a veryconvenient source of energy for remote sensing.The sun's energy is either reflected, as it is forvisible wavelengths, or absorbed and then reemitted,as it is for thermal infraredwavelengths. Remote sensing systems whichmeasure energy that is naturally available arecalled passive sensors. Passive sensors canonly be used to detect energy when the naturallyoccurring energy is available. For all reflectedenergy, this can only take place during the timewhen the sun is illuminating the Earth. There isno reflected energy available from the sun at night. Energy that is naturally emitted (such asthermal infrared) can be detected day or night, as long as the amount of energy is large enough to be recorded. Active sensors, on the other hand, provide their own energy source for illumination. The sensor emits radiation which is directed toward the target to be investigated. The radiation reflected from that target is detected and measured by the sensor. Advantages for active sensors include the ability to obtain measurements anytime, regardless of the time of day or season. Active sensors can be used for examining wavelengths that are not sufficiently provided by the sun, such as microwaves, or to better control the way a target is illuminated. However, active systems require the generation of a fairly large amount of energy to adequately illuminate targets. Some examples of active sensors are a laser fluorosensor and a synthetic aperture radar (SAR).
b) Graphic representations and geographical space can be presented in raster form or vector form In the raster format, the graphic is represented as a combination of individual “units”, where each unit can represent only one value. All units are stored to represent the graphic • Example: bitmaps of images, where the image is composed by the combination of individual pixels In the vector format, the graphic is represented by a set of points, joined by a certain relationship or function. Only the points and the relationship are stored. Intermediate points are determined
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using the relationship • Example: A CAD drawing (engineering drawing) In GIS, vector and raster are two different ways of representing spatial data. However, the distinction between vector and raster data types is not unique to GIS: here is an example from the graphic design world which might be clearer. Raster data is made up of pixels (or cells), and each pixel has an associated value. Simplifying slightly, a digital photograph is an example of a raster dataset where each pixel value corresponds to a particular colour. In GIS, the pixel values may represent elevation above sea level, or chemical concentrations, or rainfall etc. The key point is that all of this data is represented as a grid of (usually square) cells. The difference between a digital elevation model (DEM) in GIS and a digital photograph is that the DEM includes additional information describing where the edges of the image are located in the real world, together with how big each cell is on the ground. This means that your GIS can position your raster images (DEM, hillshade, slope map etc.) correctly relative to one another, and this allows you to build up your map. Vector data consists of individual points, which (for 2D data) are stored as pairs of (x, y) co-ordinates. The points may be joined in a particular order to create lines, or joined into closed rings to create polygons, but all vector data fundamentally consists of lists of co-ordinates that define vertices, together with rules to determine whether and how those vertices are joined. Note that whereas raster data consists of an array of regularly spaced cells, the points in a vector dataset need not be regularly spaced.
In many cases, both vector and raster representations of the same data are possible:
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