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| Subject: | RE: GISList: cell size for vector to grid conversion |
| Date: |
08/02/2001 12:37:55 PM |
| From: |
Neil Havermale |
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A rule of thumb gained from hard knocks via forestry GIS is a 10m grid
essentially provides equal area calculations as vectors at forest management
scales or 1:24,000 toposheets. For the USDA Soil Surveys (1:24,000 and
1:15,000) the MMU (minimum mapping unit) is held (well sort of) to 10 acres
for the similar reason.
In our studies of crop yield mapping which essentially provides "population"
data or a sample every 1/200th of an acre is a different problem. We needed
to summarize the data a bit (cut the file sizes by 3/4s or better with out
loss of information) and took a look at field variance versus variance of
nested grids - 640 acres, 160 acres, 40 acres, 10 acres, 2.5 acres, 0.625
acres, 0.15625 acres and 0.039.
Depending on the field and the nature of the crop's variation in the
landscape we found that some areas of the field could maintain a variance
distribution nearly equal to the field's at grids as large as 40 acres in
certain sections (640 acres). What we found is a very interesting variance
curve that given the objectives of study could be used to select optimal
grid size to capture variance. Our goal was to try to lower costs of
detailed soil sampling to see if we could isolate sources of the crop
variation.
Sorry I am getting a bit lost - population statistics versus sampled. If
any of you are interested I can share our method. The variance curves were
very interesting and had several curve types - sort of told us the nature of
the field's variation. But few of you will be able to gain access to
population data?
FYI
MidNight
----- Original Message -----
From: "Chris Garrard" <chrisg@biology.usu.edu>
To: "GISLIST (E-mail)" <gislist@geocomm.com>
Sent: Wednesday, 01 August, 2001 6:04 AM
Subject: GISList: cell size for vector to grid conversion
Hi all,
I'm looking for some references (peer-reviewed stuff, mostly) that
discuss vector to grid conversion. I'm especially interested in
choosing the appropriate cell size to minimize data loss, and choosing
the optimum cell size to balance data loss with storage space.
I've seen a reference to Piwowar (87) that states that the cell size
should be 1/4 the size of the minimum polygon, but I haven't yet been
able to get my hands on Piwowar's thesis. Does anyone know exactly what
this means? Do you use a cell size that is 1/4 the area of the smallest
polygon, or 1/4 the "diameter" of that polygon, or what?
I'd appreciate any comments or references.
Thanks,
Chris
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