[Xastir] Datum conversions

[James Ewen]

> > 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
> 
> What you really mean is that they're in some projection other 
> than lat/lon.

Umm.... I really don't know now! I did some reading about datum vs.
projection, and now I'm more confused than ever.
http://biology.usgs.gov/geotech/documents/datum.html

Where to lat and long enter into this mess? Is that part of the projection?
UTM uses easting and northing, and UTM is a projection. Can UTM be projected
onto different datums?

Anyway, no matter whether it's a datum or a projection problem, the numbers
that are associated with my files aren't what Xastir understands.

> The best and most certain way to determine it is to look for 
> a file that came with the shapefiles that might tell you --- 
> for example, if you have a set of shapefiles that just happen 
> to have an extra file with a ".prj"
> extension you might be in luck.  Depending on what program 
> produced the .prj "ogr2ogr" might be able to read it directly.

Hey, c'mon now... I know I ask lots of stupid questions, but I did look for
both a .prj file, and metadata before asking this time. No such luck.

> Barring that, you need to go to the source for certain.

That I have done. I was able to get a copy of the data from a local
government worker. I've sent a query back asking if the .prj or metadata is
available. 

> No, one of the problems with shapefiles is they have no way 
> of storing the projection and datum information.  The only 
> way to get this information is through auxiliary files, 
> called "metadata".

According to the webpage referenced above:

>Other software packages such as ERDAS Imagine,
>PCI, ENVI, or GRASS also allow the user to query
>the dataset to determine the datum/projection.

DO they mean that the software queries the dataset by looking at the .prj
file, or by actually looking at the data in the .shp/.shx/.dbf files?

> If you have a shapefile that is just one polyline, it would 
> not be too difficult to write a small program that converts 
> it into a file that one of the GPS upload utilities uses.  At 
> the moment, xastir has no capability to load anything *into* 
> a GPS through gpsman, only download stuff from one.

It looks like GPSMan would do the conversion, however it won't run on Cygwin
from what I've read. It looks like I might be able to find Windows based
software to do the job as well.

This is a good thing in another way as well. I've thought about using the
GPS record trail function to allow me to create a pseudo map. By carefully
selecting a limited amount of data from the shapefiles, I should be able to
create a low detail map of my local area, and upload it to the GPS as a
trail. By turning record trail off, my "map" will stay in memory. I would be
able to get a better idea where my uploaded APRS waypoints are on the GPS
map screen.

More 2 AM nights are foreseen!

James

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