How to add new a geodetic datum to Irricad

[Dan Crosby]

Irricad works internally in planar coordinates. When data needs to be geo-referenced - import from, or export to, Google Earth for example - an appropriate geodetic datum must be selected in order to convert from the planar data to geographic coordinates (latitude and longitude). Irricad comes with some common datums pre-defined. These can be accessed through Settings|Grid/Origin/GIS.

If the datum you require is not present, it can be added by editing the GeoDat.ini file found in your installed IRRICAD Config folder, commonly located in C:\Program Files (x86)\Irricad\Irricad Pro xx\Config or C:\Program Files (x86)\AEI Software\IRRICAD Pro xx\Config. The entries in that file take the following form:

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[NZGD2000/UTM]
Description="New Zealand Geodetic Datum 2000"
UTM="+proj=utm +zone=%d +south +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs"
Coord="+proj=longlat +datum=WGS84 +no_defs"

• [NZGD2000/UTM] - a short description that appears in the datum combo box
• Description - a longer description shown in the drop-down list
• UTM - the data identifying the source datum parameters - this is the important bit!
• Coord - for compatibility with the Google Earth Import/Export functions, this line tells Irricad to convert from the source datum to WGS84. This line should always be the same (as here).

A good place to find the conversion parameters for your required datum is https://spatialreference.org/, where you can search for the datum by its 'local' name (e.g., NZGD2000) or its EPSG code (if you know it). For example this link is for EPSG 2106 (NZGD2000 Bay of Plenty). From that page, click on the 'Proj4' link to get the relevant UTM conversion parameters. In this case:

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+proj=tmerc +lat_0=-37.76111111111111 +lon_0=176.4661111111111 +k=1 +x_0=400000 +y_0=800000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs 

This can then be used to create a new entry for your GeoDat.ini file, e.g.,:

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[NZGD2000/BOP]
Description="New Zealand Geodetic Datum 2000 / Bay of Plenty"
UTM="+proj=tmerc +lat_0=-37.76111111111111 +lon_0=176.4661111111111 +k=1 +x_0=400000 +y_0=800000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs"
Coord="+proj=longlat +datum=WGS84 +no_defs"