Given the interest in precise geolocation discussed below, I attach a pdf
of an article that deals with that. I'll send another one on a subsequent
message.
David
At 09:02 AM 5/6/04, you wrote:
>While I am by no means the voice of authority on these matters, I do have
>a certain amount of experience, having grappled with the same
>considerations in our study of the Cape Sable Seaside Sparrow (see
>references below).
>
>First of all, let me congratulate you on having got hold of Ikonos
>data. Like Phil Nott, I would like to know how you did that without
>leaving behind an arm and/or a leg.
>
>Secondly, the matter of spatial autocorrelation is simply unavoidable in
>almost any ecological research. Good habitat is almost always spatially
>autocorrelated; that is, if an ecological event takes place in a location
>with certain characteristics, there is usually a high probability nearby
>locations possess the same or highly similar characteristics. In our
>case, we usually found sparrows in pixels surrounded by pixels with the
>same spectral, vegetative and hydrological properties. In the case of
>validating a signature, I don't think it is a problem as the choice of a
>signature is mainly a spectral problem, not a spatial problem.
>
>I think the way you collected your training samples will stand up to
>scrutiny. I do think you need to be careful in properly locating your
>sample points, both in space and on the imagery. Using such high
>resolution imagery means you need location data accurate to within about
>10 cm. This can only be accomplished using differential corrections to
>your GPS readings. Similarly, I would not trust the rectification of the
>imagery as it comes to you. I suggest you do a very careful
>re-rectification using carefully GPSed locations of your ground control
>points. Again, you need to get your rms error down to centimeters or
>below. Otherwise, you will not be sure that a pixel on the image
>corresponds to the tree located on the ground.
>
>I infer from your message that the location data comes from USFS data. In
>other words, somebody else did it. I would give their methodology a very
>careful look and then resurvey a representative sample in order to satisfy
>myself that the data are accurate to the precision you need. I find that
>ecologists seldom understand the limitations of GPS data. They tend to
>believe the claims that are written on the box. Careful calibrations are
>absolutely necessary.
>
>I think there is no question that partitioning your data into a training
>set and an evaluation set is legitimate. In fact, you are fortunate in
>having so much data to work with. It opens up a great many possibilities
>for additional analysis. You can partition the data randomly a number of
>ways, use one partition as the training sample and see how well the
>classifier does on the other partition. I suspect you will find that
>there will not be much difference from one replication to the next. If
>there is, you will know that there is something weird going on and you can
>do a deeper investigation.
>
>I further infer that you are doing an exhaustive species map. If not, you
>need to be careful in using the maximum likelihood decision rule as it
>tends to classify every pixel as one thing or another. Also, as a matter
>of personal interest, I would like to know what you used for your
>priors. A reply off list would be appreciated.
>
>Cheers,
>
>Bob
>
>
>References:
>
>2003. Jenkins, C.L., R.D. Powell and S.L. Pimm. Demonstrating the
>destruction of the habitat of the Cape Sable Seaside Sparrow. Animal
>Conservation, 6:29-38
>
>2003. Jenkins, C.L., R.D. Powell and S.L. Pimm. Why sparrow
>distributions do not match model predictions. Animal Conservation, 6:39-46
>
>
>
>
>
>
>Robert D. Powell
>Congress Farm Research Institute
>Wilmington, Ohio, USA
>bob_powell@erinet.com
>
>Ludere cum sacris
*****************************
David B. Clark
Research Professor of Biology
University of Missouri-St. Louis
and
La Selva Biological Station
Puerto Viejo, COSTA RICA (residence)
Tel. 506-766-6565-ext. 146
Fax. 506-766-6535
Surface mail:
Organization for Tropical Studies
Interlink 341
P.O. Box 02-5635
Miami, FL 33102
USA
Address for Express Mail (FedEx,UPS,DHL):
Organizacion para Estudios Tropicales (OET), 450mts al Oeste del Colegio
Lincoln,Moravia. Frente a Servicentro Los Colegios. San Jose, Costa Rica
(tel.506-240-6696)
"Trees are sanctuaries. Whoever knows how to speak to them, whoever knows
how to listen to them, can learn the truth. They do not preach learning and
precepts, they preach, undeterred by particulars, the ancient law of life."
- Hesse
*****************************
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