On Thu, Mar 03, 2011 at 06:15:39AM -0800, we recorded a bogon-computron collision of the <[email protected]> flavor, containing: > On Wed, 2 Mar 2011, Michael Perdue wrote: > > > Hate to be a wet blanket, but both NAD27 and NAD83 UTM coordinates are > > supposed to be in meters. > [SNIP] > > Since the data have both lat/lon and some version of UTM, I think the best > approach is to use the lat/lon columns to import the data, then v.proj to > the common Albers Equal Area location.
Except that your data columns for lat/lon have far less precision than UTM, and as I pointed out in another mail, the lat/lon values at low precision convert to different UTM coordinates than the columns in the table (regardless of datum) -- hundreds of meters different, in fact. On the other hand, the much more precisely-specified UTM coordinates (expressed at centimeter precision) do in fact convert to the given lat/lon coordinates if one assumes proper rounding and NAD83 datum. It looks like the lat/lons are not the ones to trust. If you were to use the lat/lon pairs to import and convert to AEA, you're likely to have some pretty significant errors (hundreds of meters). If I were in your shoes, I'd run a test: assume NAD83 datum and use proj.4 to convert each pair of UTMs in your table to lat/lon, then round to two decimal places and compare to your lat/lon columns. If they all agree then it's a safe conclusion that the UTM coordinates are the reliable, precise ones and the datum is NAD83. Then your best bet would be to use the UTM columns to import the data into a UTM Zone 11, NAD83 location, and v.proj *those*. -- Tom Russo KM5VY SAR502 DM64ux http://www.swcp.com/~russo/ Tijeras, NM QRPL#1592 K2#398 SOC#236 http://kevan.org/brain.cgi?DDTNM "The truth will set you free, but first it will piss you off." _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
