[Xastir] Datum conversions

[Gerry Creager n5jxs]

I'm literally standing at a gae at Newark, returning from Doha, Qatar. 
I haven't worked thru e-mail in a day or so, and I'm not exactly at my 
freshest, but I'll take a crack at some of this.  I reserve the right to 
claim I was abducted  by aliens and never made any comments if they look 
like I'm sleepwalking...

James Ewen wrote:
> Okay, now you get to drag me around another learning curve.
>  
> I got ahold of a bunch of shapefiles for an area where we will be using APRS
> to track assets for an upcoming cross-country ski race in February.
>  
> I stuffed them into my map directory, and found that I could not see them.
> After a little head scratching, I finally figured out that the datum might
> be wrong. shpinfo show me file bounds of 1369668.166, 923382.659 /
> 1386313.311, 934944.981

When you got them, did they come with a '.prj' file?  That's where the 
datum and projection info would belong in a shapefile.  Unfortunately, 
not everyone does that.  It's 'metadata' and that's a dirty word to a 
lot of GIS and cartography types who don't really understand what 
they're doing.

Hmmm. This brings up an interesting point.  I'll try to remember to get 
to it later...

> I would guess that this might be UTM values, but don't know how to determine
> that for sure.

OK.  So it might be UTM, or it might not.  It *is* in an easting, 
northing coordinate system and the values appear to be in meters, but 
there's no guarantees.

However:  UTM is a *PROJECTION*, not a datum.  Datum would be something 
like Polkovo 1942, Gauss-Kreuger ellipsoid.

The Universal Transverse Mercator projection divides the Earth's 
surface, generally, into parcels 6 deg wide, divided at the equator, and 
  typically too distorted to use past some arbitrary point (42 deg N & 
S, but that's what I remember and can't document at this time...).

> Reading Tom's tutorial, it looks like I have to be able to tell the
> conversion program what datum it starts in, and what datum I'd like it to
> end up in.

Yep.

> I would guess that there's no info encoded into the file to determine what
> datum is used, otherwise the conversion program would only need to know the
> desired output format.

Not if it's lacking the .prj file.

> The area in question is at about 53.5N, 112.8W.

With luck I'm about to get on an airplane 2 hrs early for a spot closer 
to home.  I'm gonna have to clear outta here!  I'll try to check tonight 
when I can't sleep!

> Another thing I'd like to know, once I get the shapefiles working, is
> whether there is a was to convert a shapefile to a gps track. I have a
> shapefile theme that is the trace of the course to be skied. If I could turn
> that into a GPS track, and upload it to my Garmin GPS II+, I could use the
> trackback simulator to create a pseudo-lead skier.

There are tools in gdal, IIRC, and some scripts from the Mapserver folks 
to turn a shapefile into a tabular form for database use.  That's 
similar but if there's something direct to a waypoint file I'm not 
familiar with it.

> It's a little tough to run a lead vehicle in front of the leader of this
> race. Nothing is allowed on the course before the skiers. A snowmobile would
> destroy the tracks that are set for the skiers, and would cause a great deal
> of problems. The only way to track the lead skier is to give them a tracker
> to carry, or to run a simulated tracker along the course.

Sounds like a tracker would be the easiest.

73, gerry
-- 
Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University	
Cell: 979.229.5301 Office: 979.458.4020
FAX:  979.847.8578 Pager:  979.228.0173
Office: 903A Eller Bldg, TAMU, College Station, TX 77843