[aprssig] Re: aprs-scs update

KC2MMi kc2mmi at verizon.net
Sun May 8 14:39:45 EDT 2005

 Just beware the limits of TIGER data. As the Census Bureau themselves
state, that data is for demographic mapping only and it is not intended or
fit for street-level navigation and other uses. The problem is that they use
multiple data sources, including "send someone with a car to measure how
long that road is". This year they corrected the data for Queens County (NY)
but not the other four counties in NYC. The problem is that they have
previously used a NYC-only datum that goes back to colonial times--not
WGSxx. So now the 2003 TIGER maps for one county in NYC may be corrected,
while others are known to be wrong by 1/4 mile. Go back to the 2002 maps
(which are still mirrored in several online archives) and you'll be off by
1/4 mile in Queens County. Use the 2003, and Queens County will be off 1/4
mile from the neighboring counties.
 And it isn't just a NYC/NYS matter, these errors on similar scales apply to
patches all over the country. The Census Bureau are aware of this and trying
to make corrections, but they have no budget for the task. TIGER is arguably
better than nothing--but APRS users who expect "GPS quality" accuracy should
not be using the TIGER maps without prior warning. In some areas, the TIGER
maps are WGSxx clean. In others, they are not. They should not be
represented as suitable for GPS navigation systems without ground
confirmation in each county. If you're going to distribute them or endorse
the use of them, PLEASE! make sure that warning goes out with them. Along
with the note to check for annual updates, which change the datum in dozens
to hundreds of counties.

More information about the aprssig mailing list