Hello Thierry
Le 31/10/2019 à 20:08, Thierry Danard a écrit :
Where does the Apache SIS library get this “AB_CSRS.DAC” file from,
and what does it do?
This file is specified in the EPSG database; Apache SIS does not
maintain any list of datum shift files on its own. I found where this
file is used by executing the following SQL statement in the database:
SELECT * FROM "EPSG"."Coordinate_Operation" AS CO INNER JOIN
"EPSG"."Coordinate_Operation Parameter Value" AS CP
ON CO.coord_op_code = CP.coord_op_code WHERE param_value_file_ref =
'AB_CSRS.DAC';
I got 4 coordinate operations, two of them being deprecated. The two
non-deprecated coordinate operations are:
* EPSG::1702 — NAD83 to WGS 84 (8) — Approximation at the +/- 1m level.
* EPSG::1849 — NAD83 to NAD83(CSRS) (3) — Accuracy 1-2 metres.
For getting more information about those coordinate operations, go on
the http://www.epsg-registry.org/ web site. Click on "Retrieve by code"
tab, the enter "1702" or "1849" in the "Code" field. In the search
produces more than one result, select the row having "Transformation" in
the "Type" column and click on the "view" link on the right. Details
about the coordinate operation should appear. On the right side we can
read (in EPSG::1849 case):
Remarks: This gridded difference file AB_CSRS.DAC will need to be
renamed to AB_CSRS.gsb to run in some software suites. Formats
identical, but AB file is provincial fit only. Can be taken as an
approximate transformation NAD83 to WGS 84 - see code 1702.
Scope: Accuracy 1-2 metres.
Information Source: Geodetic Control Section; Land and Forest Svc;
Alberta Environment; http://www3.gov.ab.ca/env/land/dos/
So this file seems to be a datum shift file in the same way than ".gsb"
files. In Apache SIS case there is no need to rename the file,
"AB_CSRS.DAC" is as good as any other name. But I don't know neither
where to download it.
Regards,
Martin
