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[aprssig] Re: APRSpoint and other map sources

Curt, WE7U archer at eskimo.com
Thu Dec 2 21:40:29 UTC 2004


On Thu, 2 Dec 2004, Stephen H. Smith wrote:

> The basic problem is that electronic versions of USGS  topo maps are
> bitmaps (scans of paper maps), not vector images.  The resulting bulk is
> absolutely enormous.

Mostly true.  There are also SDTS-format versions of the DLG topo
contours.  Those are vector contours.  Xastir can overlay those on
top of other maps.  See http://www.xastir.org, Search and Rescue
Features link (on the left) for an example of what that looks like.


> Delorme's own TopoUSA! uses the USGS raw terrain database (the so-called
> DEMs - Digital Elevation Models) repackaged into a proprietary
> compressed file format. [DEMs consist of millions of triplets of numbers
> (lat,long,elevation) for points every 3 or 30 seconds of arc across the
> U.S.]  TopoUSA uses this data to generate maps that look very similar to
> standard topos (brown contour lines, green patches for lowlands,
> outlines of urban extents, etc.) from the raw data. It can also render
> them in raised-relief-looking 3D.  This non-bitmap (i.e. quasi-vector)
> way of storing the data for the map images allow the relief data for the
> entire US to be stored on only 6 CD-ROMs.

We have the possibility of supporting DEM's with Xastir in the
future.  The map library that supports this isn't fully integrated
into Xastir yet.  When it is, we'll have some fun with that stuff
too.  I doubt I'd be interested in any topo info that could be
stored on 6 CDROM's, as the detail just isn't there for my purposes.
Nice to look at though.

What I'd like to see is a repository for the SDTS contour files,
downloaded off the USGS site onto some other server.  Same for any
USGS geoTIFF topo images.  If we had them all available in one spot
and organized nicely, it'd help a large number of people that are
having trouble getting the ones they need.  The SDTS files are not
supposed to be available for free forever.  They're just there now
to drum up interest in the SDTS format.  All of that data was payed
for by U.S. taxpayers and is free for use, so creating our own
download server is just fine.

I have all of the topos for WA, and some for OR/ID/MT/HI.  I could
certainly try to upload a few of them if we had space for them.

Another problem is that state download sites for geoTIFF topo maps
often trim the borders, and convert them to a proprietary format
called MrSID, instead of the open geoTIFF format.  This makes them
harder to use in the future.  If we had a download server for the
original files we could extend their usefulness.

Perhaps one of the distributed servers that is always making the
news (for bad reasons) could be used for this sort of thing nicely.
Bittorrent perhaps?  Anyone know more about this sort of thing?

--
Curt, WE7U.   APRS Client Comparisons: http://www.eskimo.com/~archer
"Lotto:    A tax on people who are bad at math." -- unknown
"Windows:  Microsoft's tax on computer illiterates." -- WE7U
"The world DOES revolve around me:  I picked the coordinate system!"




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