[Xastir-dev] New stuff

[Curt Mills, WE7U]

On Tue, 13 Aug 2002, Gerry Creager wrote:

> OK, so I've not downloaded and reviewed the new stufff yet, but how are
> you evaluating the error with UTM?  If it's vice TIGER, the errors
> associated with the projection against a lat/lon unprojected view are
> significant.

With my GPS.  I put it in simulator mode, put in the coordinates I
want, then go to the navigation page and change to another
coordinate format.  Write it down.  Repeat.

I know they don't use the most accurate algorithms inside the GPS,
but I don't think we do in Xastir either.  I figure if it's less
than 5 meters, I won't worry about it.  105 is a bit much.


> I'll try to look at this and see where things are falling out.  I've got
> to revisit some things:  I think I can now display my local benchmark
> and monument database (MapInfo-generated SHP file) so, if it works and I
> can see stuff I have real accurate data to compare against.

You can probably display the points/lines/polygons, but unless we've
hard-coded algorithms to figure out your file and where to find the
labels, you won't see those.


> In looking at your explanation, however, a thought just hit.
>
> UTM anticipates a mercator projection.  GPS implies LAT/LON unprojected
> ("geographic projection").  There whall be variance, and you're up where
> it's more accentuated.

I don't think that'd cause this problem.

Here's another datapoint:  I got permission a long time back to
convert some C-code to Perl code, and wrote Coordinate.pm from it.
The C-code did datum conversions.

Olivier then took my Perl code (much later) and converted it back to
C-code, and that became datum.h and datum.c.

The Perl code reads out the same as the GPS.  The C-code doesn't.
As I recall, the C-code used to be dead-on.  Must have been one of
my "minor" changes that broke it.

Curt Mills, WE7U                         hacker.NO_*SPAM at tc.fluke.com
Senior Methods Engineer/SysAdmin
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