[GRASSLIST:2575] Re: Multiple UTM zones

[Rich Shepard]

On Mon, 8 Oct 2001, Quantitative Decisions wrote:

> UTM works by wrapping a cylinder around the earth "sideways"; that is, so
> that the cylinder contacts the earth along a meridian rather than along the
> equator.

<snip>

Bill,

  Now that you write this, it brings back the image. Thanks.

> Yes.  That is why an answer to your original question depends on your
> accuracy requirements.

  Accuracy varies with the data. For example, elevation data are generally
the most important (for hydrologic and slope stability analyses), but the
most accurate I can be is 60-90 cm using the high-precision,
differentially-corrected GPS receiver output. For most natural resource
work, that's accurate enough.

  I've worked at places where the obsession with digitized and map output
accuracy was excessive. In most of my work, that's the case. Two examples:
wetland boundaries are transition zones that may cover several meters.
Anywhere within that zone one can legitimately place a wetland boundary flag
and be equally accurate. So, the accuracy of mapping the boundary is much
less that that produced by a sub-survey grade GPS receiver. Also, animal
nests or sightings do not need to be recorded to centimeter accuracy. That
spotted owl pair (or bald eagle or what-have-you) could just have easily
nested in any tree in the vicinity.

> A lot of people do this.  However, if you are in a situation where your
> study area spans two or more UTM zones, then either (a) you don't care

  If I had a project area that large, I'd use a different projection; one
better suited to large areas. :-) FWIW, the Oregon state GIS center uses a
modified Lambert Conformal Conic projection for all the statewide data. When
I have large area projects here, that's what I use, too.

Rich

Dr. Richard B. Shepard, President

                       Applied Ecosystem Services, Inc. (TM)
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