[Proj] Proj4 Datum Grids

Dan Crosby dan.crosby at lincolnagritech.co.nz
Mon Mar 19 17:15:55 EST 2018

Hi all,
I’m just looking for some clarifications as to what I can do with Proj, bearing in mind GIS related activities aren’t my bread-and-butter. We have a CAD based product (i.e., planar) that operates, when necessary, in WGS84 UTM. I previously built a GIS library of tools to assist with this and other related things, including the Proj library.
Currently we support KML import / export, and thus lat/long conversion to WGS84 UTM. However, we are having some instance of users complaining that the KML export doesn’t work when they have UTM data using a different datum (obviously!). So I’m looking in to adding support for conversions from different datums.
I suspect support for a handful of different datums will probably cover most of my users issues  - WGS84, NAD83, NAD72, NZGD49, NZGD2000 and GDA94 to start with. Do I need to include the datum grid files in my product install to make these work? (i.e., I anticipate having UTM coordinates based on these datums and needing to convert them to WGS84 lat/long for KML export)
I thought of putting the Proj strings (e.g., "+proj=longlat +datum=NAD83 +no_defs") in an ini file so that I could add new ones without recompiling, but again I’m not sure when/if a datum grid file would be required?
Am I correct in thinking that, provided the relevant ellipse is defined in Proj, I can perform any datum shift with the appropriate +towgs84 entry?
Thanks for putting this amateur straight! J
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