[aprssig] Warning: TIGER map positions

KC2MMi kc2mmi at verizon.net
Tue Nov 30 20:59:28 EST 2004

After two weeks of pursuing a question of large errors (200-250 feet) on the
TIGER (US Census) maps used in so many APRS mapping systems, and the
priceless help of several parties (especially Gerry Creager N5JXS) I was
finally informed by the Census rep that this past April 2004 they released
the 2003 TIGER files, which include *significant* changes in over 100
counties across the country.

Anyone who is or has been using TIGER maps (i.e. the default on FINDU and
APRSWORLD or on their own computers) and has been seeing significant errors
in reported position displays, is encouraged to check out
<http://www.census.gov/geo/www/tiger/tiger2003/tgr2003.html> to see if your
county is on the list of revised maps.

The nice folks at the Census also remind me to say their data is not
intended for any precision mapping purposes, they make no warranty for that,
their business is only demographics.

So...If you already know about map datums and haven't been able to figure
out why "this APRS stuff just ain't right"...here's one more source of
error, and a way to find out if it has been affecting you.

This is not to say that ALL of the Tiger maps have problems, that's not what
I've said. Just that anyone using the TIGER maps needs to confirm their
accuracy in your own area, before you believe the "red dot" on the map. In
some areas, it is confirmed that dot will be 200-250 feet off position.

One way to verify this is to use the www.acme.com map server. Look at your
reported position on the TIGER map, then go to ACME and look at it on the
USGS TOPO map. If the two change, believe the TOPO.

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