The boundcrs.dat file associates (links) Coordinate Reference Systems to datum transformations.
Transformation datums are defined in datum.dat or epsg.db.
The format in boundcrs.dat is shown below:
CRS_ID,EPSG-CRS-ID,DatumTxID
where:
• CRS_ID is an identifier for the map definition lookup key.
• EPSG-CRS-ID is the EPSG identifier for the coordinate reference system.
• DatumTxID is the identifier for a datum transformation. N/A can be entered for no transformation.
A CRS can have multiple entries with different transformations. For example, two entries can have the same CRS_ID and EPSG-CRS-ID, but one has a DatumTxID and the other has N/A. Either the CRS with a transformation or the CRS with no transformation can be selected for a dataset.
Entries from boundcrs.dat are shown below:
[UTM - WGS84] |
UTM-01N,EPSG:32601,WG84 |
UTM-02N,EPSG:32602,WG84 |
UTM-03N,EPSG:32603,WG84 |
UTM-04N,EPSG:32604,WG84 |
UTM-05N,EPSG:32605,WG84 |
UTM-06N,EPSG:32606,WG84 |
Concatenated transformations
Concatenated transformations are sequential transformations (or conversions) between multiple coordinate reference systems.
These transformations are required to apply time-based velocity grids. Velocity grids take into account the movement of the Earth’s crust in order to apply time-based transformations. Like all transformations, the end result are coordinates in WGS 84. Time-based transformations are performed based on the data epoch value of the coordinate reference systems. If no epoch is specified, the reference epoch of the CRS is used.
The epsg.dbf file contains many predefined concatenated transformations. You can add custom concatenated transformations by inserting transformation datums separated by pipe (|) characters.
For example, see the following the following entry:
NBST83, EPSG:2953, EPSG:8676|EPSG:8264|EPSG:7666
where these operations occur in sequence:
• Canada velocity grid v6 (EPSG:8676 in the latest EPSG registry).
• 14-parameter transformation, NAD83 (CSRS) v6 to ITRF2008 (EPSG:8264 in reverse)
• 7-parameter transformation, ITRF2008 to WGS 84 (G1762) (EPSG:7666 in reverse)
The above example uses a velocity grid. Velocity grids from outside sources can be stored in the GridShifts folder (see Grid Transformations for move information).