To add the definition once so it is globally available to all users (and if you have write-permission to the file), the new definition would be added to $PCIHOME/etc/datum.txt.
Or the new definition may be added to a local copy of the file. (This would not be globally available to all users.) At the operating system level (on a Unix platform in this example), you could copy the files as follows:
% cp $PCIHOME/etc/datum.txt . % chmod 644 datum.txtNOTE: A local copy of datum.txt will be searched for before (and so has priority over) $PCIHOME/etc/datum.txt.
To define the new datum, add the new datum definition as one line of text to the datum.txt file. For example:
"D801","ATS77","E910",0,0,0,"New Brunswick",0,0,0,0The fields of the new 3 term Molodensky transform datum record are as follows:
1. "D801" - A unique code for the datum.
This would be one that doesn't exist already
in this file. The first character would be
D (or d) for Datum followed by an integer
(maximum of 3 characters).
2. "ATS77" - A descriptor string for the earth datum.
This descriptor isn't used at present in the software
but a field is expected. An empty character string ""
could be used.
3. "E910" - Field 1 of the ellipsoid record the datum uses in
ellips.txt. If this is a new ellipsoid that is
not supported at present, it may be defined as
described in the NEW ELLIPSOIDS topic.
4. 0 - X offset of the ellipsoid to the earth center in metres.
5. 0 - Y offset of the ellipsoid to the earth center in metres.
6. 0 - Z offset of the ellipsoid to the earth center in metres.
7. "New Brunswick" - A descriptor string for the earth datum.
This descriptor isn't used at present in the software
but a field is expected. An empty character string ""
could be used if you wanted.
8. 0 - Minimum error for the X dimension in metres.
This is not used and can be set as 0.
9. 0 - Minimum error for the Y dimension in metres.
This is not used and can be set as 0.
10. 0 - Minimum error for the Z dimension in metres.
This is not used and can be set as 0.
11. 0 - Number of Doppler stations used to model the offsets
and errors. This is not used and can be set as 0.
The preceding fields (1. to 10.) are required both for a new 3 term
Molodensky transform or a new 7 term Bursa-Wolf transform. A new 7
term Bursa-Wolf transform will require an additional 4 terms (12. to 15.)
added to the end of the new datum record. For example:
"D802","ATS77","E910",0,0,0,"New Brunswick",0,0,0,0,0.001,0.002,0.004,1.00001The fields of the new 7 term Bursa-Wolf datum record are as follows:
1. to 11. are the same as described above for a 3 term Molodensky transform. 12. 0.001 - X rotation in decimal seconds. 13. 0.002 - Y rotation in decimal seconds. 14. 0.004 - Z rotation in decimal seconds. 15. 1.00001 - The new scale.