# [GRASS-user] Determining Datum From Projection Coordinates [SOLVED]

Tom Russo russo at bogodyn.org
Thu Mar 3 15:34:01 EST 2011

```On Thu, Mar 03, 2011 at 06:15:39AM -0800, we recorded a bogon-computron collision of the <rshepard at appl-ecosys.com> 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."

```