[PROJ] Switch to proj-datumgrid-geotiff for PROJ 7 ?

[Even Rouault]

Nyall,

> Could we possibly keep the api returning the grid urls from the
> existing repos, mark that api as deprecated, and then add new api to
> get the new geotiff download urls? 

That would not be only at the API level. We would also need to keep for each 
grid in the database a location for the old package, and for the new API. That 
doesn't sound very good to me that we set in-stone past behaviour.

> Otherwise this is a functional
> break for clients, and I'd rather it became a hard-compilation failure
> then a silent change in behaviour.

Technically, this is not an API break. The PROJ 6 API already has a provision 
for this case.

If you look at

int PROJ_DLL proj_coordoperation_get_grid_used(PJ_CONTEXT *ctx,
                                               const PJ *coordoperation,
                                               int index,
                                               const char **out_short_name,
                                               const char **out_full_name,
                                               const char **out_package_name,
                                               const char **out_url,
                                               int *out_direct_download,
                                               int *out_open_license,
                                               int *out_available);

then when *out_package_name might actually be an empty string, and *out_url is 
then mean to be the URL of the grid to download, and not the URL of a package.

The PROJ6 database has actually already 2 entries following this for the 
Netherlands / RDNAP grids, such as

INSERT INTO grid_alternatives(original_grid_name,
                              proj_grid_name,
                              proj_grid_format,
                              proj_method,
                              inverse_direction,
                              package_name,
                              url, direct_download, open_license, directory)
                      VALUES ('rdtrans2008.gsb',
                              'rdtrans2008.gsb',
                              'NTv2',
                              'hgridshift',
                              0,
                              NULL, -- package name
                              'https://salsa.debian.org/debian-gis-team/proj-rdnap/raw/upstream/2008/rdtrans2008.gsb',
                              1, -- direct download
                              0, -- non-freely licensed. See https://
salsa.debian.org/debian-gis-team/proj-rdnap/raw/master/debian/copyright
                              NULL);

The output of
"""
#include <stdio.h>

#include "proj.h"

int main(void) {

    PJ_CONTEXT *ctx = proj_context_create();
    auto op = proj_create_from_database(ctx, "EPSG", "7000",
                                            PJ_CATEGORY_COORDINATE_OPERATION, 
true,
                                            nullptr);
    const char *shortName = nullptr;
    const char *fullName = nullptr;
    const char *packageName = nullptr;
    const char *url = nullptr;
    int directDownload = 0;
    int openLicense = 0;
    int available = 0;
    proj_coordoperation_get_grid_used(
                  ctx, op, 0, &shortName, &fullName, &packageName, &url,
                  &directDownload, &openLicense, &available);

    printf("packageName = '%s'\n", packageName);
    printf("url = '%s'\n", url);
    printf("directDownload = %d\n", directDownload);

    return 0;
}
"""

with PROJ 6.3 (and earlier) is

packageName = ''
url = 'https://salsa.debian.org/debian-gis-team/proj-rdnap/raw/upstream/2008/
rdtrans2008.gsb'
directDownload = 1


So, although admitedly the exception currently, the behaviour I propose in the 
document and that would become the general rule, can already happen.

Even

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