Does anyone know the precise list of differences between ESRI WKT and OGC WKT projection format strings?
I know there are various tools to help convert from ESRI WKT to OGC WKT, including GDAL utilities and various website services. But my question is not of a practical kind, I simply want to understand the formatting/syntax differences that these services make use of. Previous Stackexchange questions have only talked about the difference in specific examples or about the tools and services available.
Even if you just know one difference it would be great if you could just post it. From my own experience there should only be a small handful of differences. The differences i know of are:
- most text items in esri definition uses underscore where ogc uses space.
- the text defining the datum in esri wkt is same as ogc wkt except it starts with "D_".
- sometimes the text identifiers for some predefined PROJCS, PROJECTION, GEOGCS, and DATUM are written differently (eg "NAD83" in one being "North_American_1983"). I guess the only way to know which identifiers are spelled differently would be to have a list or lookup table, so please name any that you know are different.
- the various PARAMETER text values are all the same, except ogc has each word upper titlecase whereas esri has everything lowercase. However, i have seen cases where this rule hasnt been used, does anyone know if titlecase actually matters when it comes to software trying to load them?
- the type of UNIT is spelled upper titlecase in ogc and lowercase in esri, eg "Degree" vs "degree". In some cases i have seen ogc being spelled as both "meter" and "m" for "Meter" and in other cases with french spelling "metre". Anyone know what the correct convention is for these or any other unit type for both formats?
Best Answer
You've caught a lot of the differences. Esri never adopted the WKIDs for the map projection algorithms or parameter names so those are all different. We didn't agree with how carefully defined the parameter definitions are. Ours are more generalized.
We don't support TOWGS84 nor some of the newer keywords.
When we compare strings (names), we ignore the underscores, the GCS_ and D_, and case. That may not be true in other parsers. Our parser is strict on the names, but we've added some synonyms and now maintain lists of names from various vendors for comparisons.
The original coordinate system specification from OGC didn't get specific when it came to names of objects. There's a new OGC/ISO specification, "Geographic Information - Well Known Text for coordinate reference systems standard," making its way through the process towards standardization. It's much more specific about what the names should be (match EPSG registry!). It will be quite exciting implementing this standard in the future.
Disclosure: I work at Esri, am a member of the subcommittee that maintains the EPSG registry, and was a member of the CRS WKT 2.0 draft committee.