Coordinate System

A coordinate system is a reference system used to represent the locations of project data within a common coordinate framework. A coordinate system can be projected (X, Y), geographic (latitude-longitude), or geocentric (X, Y, Z). Each coordinate system is defined by a Well-Known Text (WKT) definition string, a simple structured, text-based format for spatial reference system information that is easy to store and share between systems (See http://portal.opengeospatial.org/files/?artifact_id=999 and https://en.wikipedia.org/wiki/Well-known_text_representation_of_coordinate_reference_systems). A coordinate system can optionally include a vertical datum that defines the origin for height values.

All project layers must be displayed in the terrain coordinate system. When you add a new layer to a project, TerraExplorer tries to read its coordinate system. If TerraExplorer is able to do so, it automatically reprojects the layer to the project’s coordinate system. If the layer does not include coordinate system information or TerraExplorer is unable to read this information, you can indicate the coordinate system of this source, as provided by the data supplier, in the Coordinate System dialog box. TerraExplorer then converts the layer data to the project’s coordinate system. Although the source file retains its original coordinate system, it is placed on the terrain according to the reprojection parameters.

Coordinate System Dialog Box

 The Coordinate System dialog box is a high-level dialog box that allows you to select, view, and edit a coordinate system. You can also use this dialog box to define new coordinate systems that are not already defined. This dialog box is accessed in the following cases:

§   The coordinate system of a project’s base MPT is reprojected from the Project Settings, Terrain Database tab. See “Terrain Database” in the “Setting the Project” chapter for information.

§   The coordinate system of a feature or raster layer is changed from the layer’s property sheet (In the Projection Group, in the Coordinate System field, click Set.).

§   An imagery or elevation layer does not have coordinate information or TerraExplorer cannot read it from file - in this case, the Coordinate System dialog box directly opens.

§   A navigation map’s coordinate system is changed from the Navigation Map Settings dialog box. See “Reprojecting a Navigation Map” in the “Navigation Map” chapter for information.

Coordinate System Dialog Box

 To select a coordinate system:

1.       Enter the coordinate system information using one of the following methods:

§   Click History and select one of the recently used coordinate systems. The last 20 coordinate systems used are saved in the History for future use.

§   In the Search field, type the EPSG code or a phrase from the Coordinate system Description, as indicated by the supplier, and click Search. Then select the required result from the search results displayed directly below.

§   In the Well Known Text section, type the WKT. The Well Known Text representation of spatial reference systems, regulated by the Open Geospatial Consortium (OGC), provides a standard textual representation for coordinate system information. You can edit an existing WKT. See "https://www.opengeospatial.org/standards/wkt-crs" for more information.

Note:   A custom coordinate system is listed with all the other coordinate systems, with an asterisk in the Custom column.

2.       If the coordinate system includes a vertical datum, click Set and select a vertical datum from the list of supported datums.

Note:   If the WKT of your coordinate system references a particular GTX file that was not bundled with the TerraExplorer installation, you can add the GTX by downloading it and placing it in the required directory. See “Adding Vertical Datum (GTX) Files” in this chapter for information.

3.       Click OK.

 To delete coordinate systems from History:

1.       Click History, and then in the list of coordinate systems, right-click the required custom system, and select one of the following:

§   Delete the selected item – To delete only the selected item.

§   Delete all the history items – To clear all coordinate systems from the History.

2.       Click OK.

Adding Vertical Datum (GTX) Files

TerraExplorer uses vertical datum grid shift files (GTX) for shifting and transforming between some coordinate systems that have different datums. Sometimes the Well-Known Text (WKT) of one of the coordinate systems in your project references a particular GTX file that was not found on your computer, or the reference itself is missing from the WKT. The steps below describe how to add the missing GTX information in both of these cases.

 To add a GTX that was not found on your computer, find and install the missing file:

1.       Generally, when one of the coordinate systems in your project references a particular GTX file that was not found on your computer, a notification will display in the coordinate system dialog box or in the Error/Message Log with a link to a page with the required GTX file. The GTX file on this page is packaged as a TEZ, a TerraExplorer zip file that enables the GTX to be installed without manually copying the file into a specific folder. In this case, download the TEZ and then simply double-click the file to install. See "Package Vertical Datum (GTX) Files" in the "Working with Tools" chapter for more information. TerraExplorer will save the GTX to %ProgramData%\Skyline\gdal_proj_data (e.g., C:\ProgramData\Skyline\gdal_proj_data). If TerraExplorer is unable to save to that folder, it will save to %AppData%\Skyline\gdal_proj_data (e.g., C:\Users\[user name]\AppData\Roaming\Skyline\gdal_proj_data).

2.       If no TEZ was made available to you with the missing GTX, see the following page with download links to many common GTX files. Download the required zip file and extract the GTX to %ProgramData%\Skyline\gdal_proj_data (e.g., C:\ProgramData\Skyline\gdal_proj_data). If you do not have the necessary permissions to copy to this path, you can also copy the GTX file to either of the following directories:

§   %AppData%\Skyline (e.g., C:\Users\[user name]\AppData\Roaming\Skyline)

§   Proj subdirectory of installation directory (C:\Program Files\TerraExplorer\proj)

Note:       Additional common GTX files are available from the following websites:

·         https://github.com/OSGeo/proj-datumgrid/tree/master/

·         https://vdatum.noaa.gov/download.php

·         https://raw.githubusercontent.com/OSGeo/proj.4/gh-pages/proj.pdf

·         https://proj.org/resource_files.html#transformation-grids

 If a reference to the GTX file is missing in the VERT_CS section of the WKT, determine what GTX file is required, and do the following:

1.       Add a reference to the required GTX within the WKT's VERT_DATUM object that is under the VERT_CS object:

EXTENSION["PROJ4_GRIDS","[Name of GTX File]"

E.g.,

COMPD_CS["RGF93 / Lambert-93 + NGF-IGN69 height",

            PROJCS["RGF93 / Lambert-93",

            ...,

            AUTHORITY["EPSG","2154"]],

VERT_CS["NGF-IGN69 height",

            VERT_DATUM["Nivellement General de la France - IGN69",2005,

                        EXTENSION["PROJ4_GRIDS","RAF18.gtx"],

            AUTHORITY["EPSG","5119"]],

            UNIT["metre",1,AUTHORITY["EPSG","9001"]],

            AXIS["Up",UP],AUTHORITY["EPSG","5720"]]

]

2.       Find and install the missing GTX file as described above.