[PROJ] How to add custom grid shift to proj.db?

[Peter Gagliardi]

Hi everyone,

Suppose I wanted to create a compound CRS using a 2D projection + custom
geoid file (let's call it foo.gtx), not included in proj-datumgrid. What
table(s) in proj.db would I have to update so PROJ would know to open the
right geoid file? I'm guessing `grid_transformation` is relevant here, but
I'm not sure what else (maybe vertical_crs?). Also, if it is possible to
update the database like this, how would I select this custom CRS on the
command line, whether as a WKT or something else?

A few months ago, I asked about how to do this in a WKT, and it helped me
create a WKT like the one in the postscript. In the example, all I'm
trying to specify is EPSG:31370 + a hBG18.gtx geoid heights (file provided
by a client, but not in the list of geoids in proj-datumgrid-europe). This
does seem to work correctly.

However, we're using this as part of a REST API, so my team's a little
concerned about the security implications of specifying the grid shift
filename in the WKT. We want to know if updating the database is a better
approach.

Note that I am using PROJ 6.3.1 right now.

Thank you,
Peter Gagliardi

PS: Here's what I'm using currently to make my custom compound CRS:

COMPOUNDCRS["User-Defined Projection", PROJCRS["Belge 1972 / Belgian
Lambert 72",
    BASEGEOGCRS["Belge 1972",
        DATUM["Reseau National Belge 1972",
            ELLIPSOID["International 1924",6378388,297,
                LENGTHUNIT["metre",1]]],
        PRIMEM["Greenwich",0,
            ANGLEUNIT["degree",0.0174532925199433]],
        ID["EPSG",4313]],
    CONVERSION["Belgian Lambert 72",
        METHOD["Lambert Conic Conformal (2SP)",
            ID["EPSG",9802]],
        PARAMETER["Latitude of false origin",90,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8821]],
        PARAMETER["Longitude of false origin",4.36748666666667,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8822]],
        PARAMETER["Latitude of 1st standard parallel",51.1666672333333,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8823]],
        PARAMETER["Latitude of 2nd standard parallel",49.8333339,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8824]],
        PARAMETER["Easting at false origin",150000.013,
            LENGTHUNIT["metre",1],
            ID["EPSG",8826]],
        PARAMETER["Northing at false origin",5400088.438,
            LENGTHUNIT["metre",1],
            ID["EPSG",8827]]],
    CS[Cartesian,2],
        AXIS["easting (X)",east,
            ORDER[1],
            LENGTHUNIT["metre",1]],
        AXIS["northing (Y)",north,
            ORDER[2],
            LENGTHUNIT["metre",1]],
    USAGE[
        SCOPE["unknown"],
        AREA["Belgium - onshore"],
        BBOX[49.5,2.5,51.51,6.4]],
    ID["EPSG",31370]], BOUNDCRS[
         SOURCECRS[
            VERTCRS["unknown",
                VDATUM["unknown"],
                CS[vertical,1],
                    AXIS["gravity-related height (H)",up,
                        LENGTHUNIT["metre",1,
                            ID["EPSG",9001]]]]],
        TARGETCRS[
            GEOGCRS["WGS 84",
                DATUM["World Geodetic System 1984",
                    ELLIPSOID["WGS 84",6378137,298.257223563,
                        LENGTHUNIT["metre",1]]],
                PRIMEM["Greenwich",0,
                    ANGLEUNIT["degree",0.0174532925199433]],
                CS[ellipsoidal,3],
                    AXIS["latitude",north,
                        ORDER[1],
                        ANGLEUNIT["degree",0.0174532925199433]],
                    AXIS["longitude",east,
                        ORDER[2],
                        ANGLEUNIT["degree",0.0174532925199433]],
                    AXIS["ellipsoidal height",up,
                        ORDER[3],
                        LENGTHUNIT["metre",1]],
                ID["EPSG",4979]]],
   ABRIDGEDTRANSFORMATION["unknown to WGS84 ellipsoidal height",
            METHOD["GravityRelatedHeight to Geographic3D"],
            PARAMETERFILE["Geoid (height correction) model
file","hBG18.gtx",
                ID["EPSG",8666]]]]]
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