moving raster maps across UTM zone boundaries

[Gerald I. Evenden]

>To: grassu-list at max.cecer.army.mil
>Date: Thu, 15 Apr 93 11:45:53 CDT
>
>Hello fellow GRASS users.
>
>	I've been fortunate in the past to have data sets well within UTM zones.
>Unfortunately, that happy period is at an end.  I am dealing with county soils
>and land use data for Oklahoma which is split between zones 14 and 15.  What I 
>want to do is bring the raster map for a county in zone 15, and patch it with
>a raster map in zone 14.  I've tried this procedure:
>
>1) used r.poly to convert the zone 15 map to a vector map.
>2) used v.out.ascii to convert the grass vector map to ascii.
>3) used m.u2ll to convert the ascii vector UTM map to lat.-lon. coordinates
>4) used m.ll2u to convert lat-lon. ascii vector map to zone 14 utm coordinates
>5) used v.in.ascii to convert new zone14 ascii vector map to grass vector map.
>
>Now, I'm stuck.  The manual hints that I should apply v.digit to re-assign
>attributes to the vector map before using v.to.rast.  BUT, I'm dealing with 
>_thousands_ of cells.  Is there a better way to do this whole process?  Or
>does anyone have any suggestions?  I suspect this question may be posted
>frequently, but I only recently began monitoring this discussion.  Thanks in 
>advance for any and all replies.
>
>Sincerely,
>
>Fred L. Ogden >Iowa Institute of Hydraulic Research
>flogden at hml.uiowa.edu

A principle factor in this problem seems to be: do you need to use UTM
and wouldn't you probably be better off in some projection system suited
to the extent of you data.  UTM is fine as a regular system for large
scale mapping problems that can be split on the boundaries of the
required zones, such as quadrangle sheets.  But, as you have pointed
out, it is not suited for many other applications.

I would rule out Oklahoma's SPCS as an alternative because you would
run into the same problem with its two, north and south zones.  Some
states use a non-surveying system for whole state maps and in this
case Lambert Conformal Conic would be appropriate.

Unless you have external factors not under your control, such as
outside requirements for UTM based data, or have the need for
maintaining high resolution of horizontal control (some of which is
going to be lost when exceeding UTM zone limits) I suggest you look
into an alternative which can cover the whole state with one
cartesian system.

I am putting this response to the mail-group as a whole, because I
feel that too often people get trapped into using projection systems
without a fair analysis of the needs of their application.  A lot of
times UTM is chosen just because the user has heard of it or
a friend used it.  A lot of grief results and often the UTM gets
bent well beyond its defined limits.

Flames, comments or questions welcome.

Gerald (Jerry) I. Evenden   Internet: gie at charon.er.usgs.gov
voice: (508)563-6766          Postal: P.O. Box 1027
  fax: (508)457-2310                  N.Falmouth, MA 02556-1027