[GRASS5] datum support in GRASS

[Rich Shepard]

On Thu, 13 Jul 2000, Andreas Lange wrote:

> Could please tell me someone if there are GRASS applications that use
> resolutions with sub-meter or meter range and interface with programs that
> use datum shifting?

Andreas,

  In my areas of expertise (ecology, biology, hydrology) there is almost
never a need for accuracies greater than ~5 meters. Then, there are the
accuracies of source data to be considered. For example, here in the US,
topographic map data (paper or digital) have an accuracy of +/- 15 meters
(48 feet). My opinion is that transformation accuracies greater than these
are not needed. The only exception might be if someone had to do a series of
transformations to run, and the positional errors cascaded through the
calculations.

  In our work, we tend to transform data once. If it's not in the
projection/coordinate system we use, then we transform the data. I cannot
recall a project where we did multiple transformations or shifting of the
datum.

> 4. The map datum defines an ellipsoid. On creation of the location the
> user will be asked to provide a datum, there will be no need to provide
> an ellipsoid. Is this ok or will this confuse users?

  Asking the user for an elipsoid in addition to a datum will probably cause
confusion. So, requesting the datum is OK, but assume the appropriate
elipsoid.
 
> One additional problem within 2D and 2.5D GIS is that the height component
> of the datum transformation is always lost/discarded, so that if you shift
> several times (e. g. from local datum to wgs84 and back) errors will creep
> in. I don't know if this is a problem in praxi, perhaps someone could
> comment on this.

  Again, our experience is to transform once, if needed. Our GPS receiver
collects positional data with horizontal accuracy up to +/- 30 centimeters
(corrected) and vertical accuracy of +/1 60-90 centimeters. It is for the
latter precision that I purchased this unit. We need to map mines and
quarries, and there is almost never sufficiently useful elevation data. I am
able to collect data using Lat/Lon, UTM or State Plane Coordinates, so
rarely do I need to transform/shift these.

  With purchased data (or other data of unknown provinance), the accuracy
may or may not be known. Again, we do not keep shifting the data so I can't
say that your approach would ever cause us problems.

Thanks,

Rich

Dr. Richard B. Shepard, President

                       Applied Ecosystem Services, Inc. (TM)
              Making environmentally-responsible mining happen. (SM)         
                       --------------------------------
            2404 SW 22nd Street | Troutdale, OR 97060-1247 | U.S.A.
 + 1 503-667-4517 (voice) | + 1 503-667-8863 (fax) | rshepard at appl-ecosys.com


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