ESRI and .prj

[Ben Discoe]

I asked ESRI about the situation with their .prj files, and got the
following excellent detailed answer, which i suspect will be of general
interest to the list. I'll try to summarize it on the VTP site.

-Ben

-----Original Message-----
From: Melita Kennedy [ESRI-Redlands] [mailto:mkennedy at e...]
Sent: Wednesday, August 22, 2001 6:31 PM
To: ben at v...
Subject: Re: FW: [vtp] Requesting input on BT and projections


Hi Ben,

I'm the map projection product specialist here at ESRI. I know
more of the metadata and how the low-level Projection Engine
is applied in our software.

We've been building a stand-alone C library to support coordinate
system definitions, projections, and transformations for several
years. We didn't want to use the old-style PRJ format because it
has limitations in what it supports and everything is hard-coded
to tables. Given a spheroid keyword, you have to look up its
values, for example. The library is called the Projection Engine.

We based our model on EPSG/POSC because at the time, they had what
we considered to be the most complete and understandable one.
We then proposed our model for OpenGIS and it was accepted.
OpenGIS is expanding their model and making it more closely match
EPSG but it's still quite recognizable.

So, any ESRI software that uses the Projection Engine also supports
the original OpenGIS WKT standard. Currently, that's ArcGIS 8
(ArcMap, ArcCatalog, parts of ArcToolbox), ArcSDE starting with
3.0, MapObjects starting with 2.0, ArcView Projection Utility,
ArcIMS 3.1.

ArcGIS/ArcInfo 8 is an odd case. It combines the old workstation
ArcInfo so grids and coverages still use the old-style PRJ files.
Desktop ArcInfo includes ArcCatalog/ArcMap/ArcToolbox. Shapefile,
ArcSDE layers, and geodatabase data all use the newer Projection
Engine definitions. Starting with 8.1, we have functions to
convert back and forth between the two. Not everything maps
exactly due to different supported projections and capabilities.

Melita

--
Melita Kennedy
Product Specialist
ESRI, Inc.
mkennedy at e...