libratory_5_ttransformation by using arcgis

8/7/2019 Libratory_5_Ttransformation by using ArcGIS

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Libratory 5: Coordinate Transformation of Hong Kong Spatial Data in ArcGISDesktop(Reference: ESRI China (Hong Kong) Limited)

Background

There are two types of coordinate systems being used in ArcGIS Desktop: Geographic CoordinateSystem and Projected Coordinate System . The former one has its coordinates measured inlatitudes and longitudes which are angles measured from the Earth’s center to a point on the Earth’ssurface; while the coordinates of the later one is measured in X and Y positions based on a grid lyingon a flat surface. They are closely related since projected coordinate systems are derived fromgeographic coordinate system through the process of Map Projections.

Geographic coordinate systems are further divided into two types, earth-centered and local. An earth-centered coordinate system has its origin placed at the earth’s currently known center of mass and ismore accurate overall. A local coordinate system is aligned so that it closely corresponds to the

earth’s surface for a particular area and can be more accurate for that particular area. Hong KongSpatial Data always tied to these two types of coordinate system which are WGS84 and HK80 , theformer one is earth-centered and is used in Global Positioning System (GPS) to calculate latitudesand longitudes while the later one is local specifically for the Hong Kong Territories. The latitudes andlongitudes measured in these two systems can be transformed to one another through the process of Datum Transformation.

However, Hong Kong Spatial Data are represented using projected coordinate system which isknown as Hong Kong 1980 Grid derived from HK80 and spatial features in this system have their Xand Y coordinates in the magnitude of 800000m. The X coordinates are referred to Easting (denotedas mE) while the Y coordinates are referred to Northing (denoted as mN).

The use of GPS as GIS data collection becomes common over past years, when transforming GPScoordinates, the following transformation path has to be followed in order to map the GPS data onHong Kong Spatial Data (such as basemap from Lands Department) under Hong Kong 1980 GridSystem:

Exercise: Transformation from WGS84 to Hong Kong 1980 Grid



1 Defining spatial reference to your data

1.1 Open ArcCatalog

1.2 Navigate to your data folder

1.3 Right click your layer and click Properties

1.4 The Properties Dialog will open; go to XY coordinate system tab

1.5 clicks select to browse for Coordinate System1.5.1 If your data is based on Hong Kong 1980 Grid , navigate to Projected CoordinateSystems\National Grids and choose Hong Kong 1980 Grid.prj for your data.

1.5.2 If your data is based on WGS84 , navigate to Geographic CoordinateSystems\World and choose WGS 1984.prj for your data.



1.5.3 If your data is based on HK80 , navigate to Geographic Coordinate Systems\Worldand choose Hong Kong 1980.prj for your data

1.6 Click Add upon finish, the details of the chosen coordinate systems will be shown in thedialog.

Click Select to browsethe coordinate system



1.7 Click OK twice to close all dialogs.

2 Defining Data Frame Properties to conduct coordinate transformation from WGS84 to HongKong 1980 Grid ( for displaying the spatial data in same coordinates system)

2.1 Open ArcMap and add your data of which the coordinate system is defined as WGS842.2 Note the status bar, the latitude and longitude will appear

2.3 Right click the Data Frame and click properties



2.4 The Properties Dialog will open, go to General Tab, select display unit as Meters and then go

to Coordinate System Tab:



2.5 In the Coordinate System Tab , expand the Predefined folder as show above, navigate toProjected Coordinate Systems > National Grids and click on Hong Kong 1980 Grid , thedetails will be shown in Current coordinate system, this selection will invoke the MapProjection process, then click Transformation button to open the Geographic CoordinateSystem Transformations Dialog, this will invoke Datum Transformation process:



2.6 Both map projection and datum transformation processes requires parameters, all theseparameters were already built-in in ArcGIS Desktop. It is not necessary to modify mapprojection parameters but for datum transformation parameters, you are advised to modifythe default values if you have acquired a more accurate set of parameters or you want tochange the datum transformation method. Click the New button to open the NewGeographic Transformations Dialog to modify Methods and Parameters:



2.7 If you just want to use the default values, click OK to close the Geographic CoordinateSystem Transformations Dialog and click OK again to close the Data Frame PropertiesDialog, the map display area will refresh and transform the data to Hong Kong 1980 GridSystem

2.8 Note that the coordinates shown in status bar has been changed to 800000m magnitudes:

2.9 You can add your basemap data to check if the transformation is correct.

3. Coordinate transformation from WGS84 to Hong Kong 1980 Grid (for creating a newcoordinate system file)

3.1 Project tool will be used for transformation. Show the ArcToolbox window by click theShow/Hide ArcToolbox Window button.



3.4 Click OK. And you can open both WGS84 and HK 1980 Grid data for checking.



3.5 There will about 310m shift if datum transformation process (Transformation button notclicked) is skipped:



Note:1. If you want to transform Hong Kong 1980 Grid data to WGS84 data, follow the above steps but in Step 2.5, navigate

to Geographic Coordinate Systems > World and click on WGS 1984, then click Transformation button to invokedatum transformation.

2. If you want to transform Hong Kong 1980 Grid data to HK80 data, follow the above steps but in Step 2.5, navigate toGeographic Coordinate Systems > Asia and click on Hong Kong 1980, it is NOT necessary to click the

Transformation button.3. If your source data is in HK80 coordinate system and you want to transform it to Hong Kong 1980 Grid coordinate

system, follow the above steps but in Step 2.5, navigate to Projected Coordinate Systems > National Grids andclick on Hong Kong 1980 Grid, it is NOT necessary to click the Transformation button.

libratory_5_ttransformation by using arcgis

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