ESRI China (Hong Kong) Limited Editing Geodetic Datum Transformation Parameters in ArcPad 7.0 when using Hong Kong Data Article ID Software Platform Date

: : : :

100008 ArcPad 7.0 All certified platforms March 31, 2006

Background ArcPad supports on-the-fly coordinate transformation from one coordinate system to another coordinate system. When using ArcPad with GPS, the coordinates from the GPS receiver are always in WGS84 format, but our local data is using Hong Kong 1980 Grid system, we can make use of this function in ArcPad 6 to avoid converting the data back to WGS84 to use with GPS. In ArcPad 6, we need to edit the apDatums.dbf file located in \Program Files\ArcPad\System to add transformation details and parameters used in Hong Kong in order to activate this function. However, in ArcPad 7.0, ArcPad contains all the projection and datum transformation information from the ArcGIS Projection Engine. This information is extracted from the ArcGIS Projection Engine, and embedded in the ArcPad.exe executable. No changes are made to the information before being included in ArcPad. Only a subset of the projection algorithms and datum transformation methods are included in ArcPad, as only some of these are appropriate for field mapping. ArcPad includes two utilities for working with projections and datums. These utilities are located in the Utilities dropdown list, which is in the Tools dropdown list: Reproject Shapefile: reprojects the selected shapefile to another projection and/or datum. Export Projection Information: exports ArcPad’s internal projection and datum information into text and CSV files, with a prefix of “Export_”. The files are exported into the System folder, under the ArcPad installation folder.

In the new configuration of ArcPad 7.0, the apDatums.dbf of ArcPad 6 does not exist anymore; instead, two new custom datum configuration files “Defaulttransforms.dbf” and “Transforms.dbf” will be used.

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ESRI China (Hong Kong) Limited Datum Configuration in ArcPad 7.0 You can view the list of supported datums and datum transformation methods by running the Export Projection Information utility. The Export Projection Utility creates a number of files in the ArcPad “System” folder, including the following: Export_Datums.csv, which lists all of the datums and the corresponding spheroids. Export_Spheroid.csv, which lists all of the spheroid parameters. Export_Transforms.csv, which lists all of the datum transformation parameters embedded in ArcPad as well as the datums included in the DefaultTransforms.dbf file. Export_TransformMethods.csv, which contains a list of all the supported datum to datum transformations built into ArcPad. Additional datum to datum transformations can be added to Transforms.dbf. Existing transformations listed in Export_Transforms.csv can also be overridden via Transforms.dbf. You should not edit these exported files, as they are not used by ArcPad. ArcPad includes two datum configuration files, Transforms.dbf and DefaultTransforms.dbf, which are located in the ArcPad “System” folder. These files can be modified to add datum transformations as well as to override the default datum transformations within ArcPad. By default these files are empty, but they include all of the fields required by ArcPad. Transforms.dbf. This file allows the user to add datum transformation parameters that are searched before the internal datum transformations. This means that the data in this configuration file will override the datum transformation information embedded in ArcPad. (This file is similar to the ArcGIS GEOTRAN table.) To modify the Transforms.dbf file, you must supply two datum names and all of the associated transformation parameters between these two datums. ArcPad will attempt to match the two names to the datum names listed in projection (.prj) files and information, and then use the associated parameters if a match is found. No datum transformation will take place if no match is found. DefaultTransforms.dbf. Sometimes there is more than one set of parameters to transform between two datums. By default, ArcPad will use the first match it finds in the internal tables embedded in ArcPad. The DefaultTransforms.dbf file forces ArcPad to use a specific datum transformation if there is more than one available for a given pair of datum names. For example, there are multiple sets of parameters to perform a datum transformation between the “D_WGS_1984” and “D_Australian_1966” datums: ‧ ‧ ‧ ‧ ‧

AGD_1966_To_WGS_1984 AGD_1966_To_WGS_1984_12 AGD_1966_To_WGS_1984_13 AGD_1966_To_WGS_1984_14 AGD_1966_To_WGS_1984_15

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ESRI China (Hong Kong) Limited By default, ArcPad always uses the first set of datum parameters for a particular datum transformation, which in this case is AGD_1966_To_WGS_1984. However, by adding “AGD_1966_To_WGS_1984_14” to the DefaultTransforms.dbf file, you override ArcPad’s default and force ArcPad to use this set of datum transformation parameters. Modify/Add Geodetic Transformation Parameters (WGS84 to/from Hong Kong 1980 Geodetic Datum) As mentioned in previous section, Export_Transforms.csv lists all of the datum transformation parameters embedded in ArcPad. You may find the WGS to/form Hong Kong 1980 Geodetic Datum Transformation information also included in Export_Transforms.csv. That means ArcPad 7 now has supported on-the-fly coordinate transformation from WGS84 coordinate system to Hong Kong 1980 coordinate system when we are using GPS without any pre-configuration as it did in ArcPad 6. Yet, user may find that the internal datum transformation parameters from WGS84 coordinate system to Hong Kong 1980 coordinate system may not be synchronized with the parameters published by Lands Department. In this case, you have to manually edit the Transfrom.dbf to add the new parameters in order to override the existing datum transformation information embedded in ArcPad. Step 1 Before you add the new datum transformation parameters, you can find the most updated datum transformation parameters in the following link provided by Lands Department of Hong Kong SAR. http://www.landsd.gov.hk/mapping/en/news/image/7p_itrf96_hk80_v1.pdf (Note: This URL is maintained by Lands Department, if you found that this URL is not accessible, please contact Lands Department for any updated parameters.)

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ESRI China (Hong Kong) Limited Step 2 Add a new record in the Transform.dbf with the following details: Table Field

Field Value

Remarks

Transform

WGS_1984_To_Hong_Kong_1980

Transformation name but not essential.

Method

Coordinate_Frame

According to the 7-P provided by Lands Department, the transformation method from WGS84 to Hong Kong 1980 is Coordinate_Frame, please refer the Export_TransformMethods.csv for other supported method.

Datum_1

D_WGS_1984

Please refer to Export_Datums.csv for datum name. ArcPad will try to match the datum name with the datum name listed in projection file.

Spheroid_1

WGS_1984

Please refer to Export_Datums.csv for the corresponding Spheroid name.

A_1

6378137.000000000

Please refer to Export_Datums.csv for the corresponding Axis value.

F_1

0.003352811

Please refer to Export_Datums.csv for the corresponding 1/ Flattening value. In this case, 1/ 298.257223563 = 0.003352811

Datum_2

D_Hong_Kong_1980

Please refer to Export_Datums.csv for datum name. ArcPad will try to match the datum name with the datum name listed in projection file.

Spheroid_2

International_1924

Please refer to Export_Datums.csv for the corresponding Spheroid name.

A_2

6378388.000000000

Please refer to Export_Datums.csv for the corresponding Axis value.

F_2

0.003367003

Please refer to Export_Datums.csv for the corresponding 1/ Flattening value. In this case, 1/ 297 = 0.003367003

Dx

162.619000000

Shift along x-axis. Refer to the most updated parameter provided by Lands Department.

Dy

276.961000000

Shift along y-axis. Refer to the most updated parameter provided by Lands Department.

Dz

161.763000000

Shift along z-axis. Refer to the most updated parameter provided by Lands

Ds

1.094239000

Scale factor. Refer to the most updated parameter provided by Lands

Rx

0.067741000

Department. Department. Rotation about x-axis. Refer to the most updated parameter provided by Lands Department. Ry

-2.243649000

Rotation about y-axis. Refer to the most updated parameter provided by

Rz

-1.158827000

Rotation about z-axis. Refer to the most updated parameter provided by

Xcr

0

X Coordinate of Rotation Origin. It’s an additional parameter used in

Lands Department. Lands Department. Molodensky-Badekas transformation method. Just left it with 0 in this case, as we don’t require it. Ycr

0

Y Coordinate of Rotation Origin. It’s an additional parameter used in Molodensky-Badekas transformation method. Just left it with 0 in this case, as we don’t require it.

Zcr

0

Z Coordinate of Rotation Origin. It’s an additional parameter used in Molodensky-Badekas transformation method. Just left it with 0 in this case, as we don’t require it.

Dataset

0

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Just left it with 0 in this case, as we don’t require it.

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ESRI China (Hong Kong) Limited Step 3 Close and re-open ArcPad if it is still running. If you run “Export Projection Information” tool again to export the projection information, you will find a new record will be added in the first part of Export_Transforms.csv file as shown in below picture:

Step 4 You can only check what datum transformation is being used between the GPS datum (usually WGS_1984) and the datum for the current map projection. 1. Open ArcPad with any map which in Hong Kong 1980 Grid projection. 2. Open the GPS Preferences dialog box, and activate the Datum page. 3. Select D_WGS_1984 as the GPS Datum

4.

Click to open the Datum Transformation Information message box, and view the datum parameters of the currently selected GPS Datum, including the parameters that ArcPad will use when transforming coordinates from the GPS datum to the datum of the map. The parameters listed correspond to the first set of parameters found in the transform.dbf, or secondly in the internal datum tables. Missing information indicates that no matching transformation parameters were found between the GPS datum and the datum of the current map projection—and so ArcPad is performing no datum transformation.

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ESRI China (Hong Kong) Limited Step 5 After you set the datum transformation, you may test it with any Hong Kong 1980 Grid projected map in the field if you have GPS with you right now or you can simply test it with the FakeGPS extension of ArcPad. The FakeGPS is an extension for ArcPad 6.0.1, 7.0 or higher that will simulate a GPS. It will read longitude - latitude points and optionally height values from a text file. It will send the points, one at a time, at a given interval, to ArcPad. The points will appear to ArcPad as if they are coming from a GPS. You may download the FakeGPS extension in ArcScript of ESRI Support Center in below link: http://arcscripts.esri.com/details.asp?dbid=13817 After you installed the FakeGPS extension in ArcPad following its instruction, you could prepare a text file with following line, this is a sample GPS coordinate in degrees, minutes, seconds with decimal seconds (DMS) 114 00 1.079932 22 29 8.777176 The Fake GPS Properties Dialog Box allows you to specify which point file to use, how to read the point file and how often the points will be sent to ArcPad. You can go to "Tools | Options | Extensions" choose "Fake GPS" and press the "Properties" button to access the Properties Dialog Box and specify the point file you just created as follow:

Then, set the GPS protocol to “FakeGPS” in the GPS Preference dialog box as follow:

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ESRI China (Hong Kong) Limited Once the FakeGPS properties have been set and the GPS activate button is pushed, points will be read from the file and passed to ArcPad as if a GPS was connected to ArcPad. The GPS tracking cursor will be displayed on the screen showing the current course over ground. The GPS position window can also be displayed and many of the fields are filled in, to simulate a GPS. The current coordinate and elevation are accurately displayed. In this case, the window will shows the coordinate as we input in the text file in DMS format as default.

Mouse click on the coordinate on the top of the window to transform the GPS coordinate to your map projection, which is Hong Kong 1980 Grid projection in our case.

If you see 818097 E, 838477N showing in the GPS position window after transformation, this means that datum transformation parameters you set in the transform.dbf are working properly.

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