Hi Adrian, Simone et al., (note: I've sanitised the subject line as it was getting out of hand!)
Thanks very much for these very valuable clarifications and full explanations. I think I understand this now and I'm comfortable that a coverage can be non-contiguous and that every point in a coverage must contain a value. This seems to be in line with the design of CSML. Thanks to all for a very useful discussion. I hope this helped others as much as it helped me! Cheers, Jon On Fri, Jun 13, 2008 at 1:41 PM, Adrian Custer <[EMAIL PROTECTED]> wrote: > On Fri, 2008-06-13 at 12:27 +0100, Jon Blower wrote: >> Hi Michael, >> >> Thanks very much for this. >> >> > My naive 2p / 2c worth is that the domain of a coverage is simply that >> > region within which data are defined. >> >> I like this definition because I understand it! However, I'm not sure >> that everyone has the same view. I think the $64000 question is: does >> the domain for a single coverage have to be contiguous? If so, this >> would seem to rule out the use of a Coverage for a discretely-sampled >> domain in which you don't want to apply interpolation of any kind. > > No, a coverage domain does not need to be continuous. A coverage, in its > most abstract, is merely: > > some set of direct positions > for all of which we have > a set of values. > > The set of direct positions may be finite (i.e. a random or regular > group of points) or infinite (i.e. a set of polygons, a mix of points, > lines and polygons). The values can be of any kind of measure: nominal, > ordinal, interval, or ratio and can also be a vector of, possibly mixed, > values. The coverage is however *required* to have a value for each > position in the original set. The key for the discrete/continuous will > be how the values are generated. > > An example of a discrete coverage might be "countryNameCoverage". The > domain of such a coverage could be the set of polygons of territories > claimed to be occupied by some nation state (i.e. in today's world, all > the land masses.) For our purposes let's call this a set of mostly > non-overlapping polygons. The coverage, by its construction, guaranttees > that for any point within those polygons we can get a "name" for a > country. So if we give it a point in Boston we get 'USA', if we give it > a point in the mississippi we get 'USA' but if we give it a point in the > Amazon, we could get 'Brazil'. The coverage can define rules about how > it resolves disputed areas like land in Antarctica. So this is > 'discrete' in that we get the same result wherever we are within a > polygon---any point we ask for within the alaska polygon will always > give us 'USA'. > > An example of a continous coverage, based on the same polygons, could be > "populationDensityCoverage". There, our two queries in the alaska > polygon could return completely different values and indeed, in general, > we expect different points to have different values even within the same > polygon. > > Do not confuse continuous coverage with the continuity of the values > however. We could have a third coverage using the same polygons which is > "sexOfclosestHumanCoverage" which would return 'female' 'unknown' 'male' > for any position in the polygons. Again every point in Alaska would have > an answer but that answer would be different for different places in > Alaska so the coverage is continuous although the values are not. > > An 'image' can be turned into a coverage in several ways although a > common one will be to characterize the domain as a single, continuous > rectangular block everywhere over which the coverage can return a vector > of values, say one value for each of the image bands. The return vector > would vary with position across the single domain so this would be a > continuous construction. Alternatively, we could define an 'image' as a > multi-domain discrete coverage where the domain is a set of equally > sized polygons arranged side-by-side and the value returned is the same > for each position within each polygon. Note in passing that we are not > talking about how the values are generated---that's a detail of the > internals of the coverage. > > In many ways coverages are the end goal of GI Systems so they are rich > and complex. Also, Geotools has for a long time mixed up the notion of > GridCoverage with the notion of Coverage causing some confusion. > > For details of the construction of these things, please look at the spec > itself. > > Also, you should all be aware there's a big doc written by Bryce trying > to play with these notions. He's good at giving a 'read' of the specs he > looks at so one can compare one's own understanding to someone else's > (ie. his) as one reads. See > http://docs.codehaus.org/display/GEOTOOLS/ > ISO+19123+progress+and+future+plan > (rebuild the link) > for details. > > Hope that helps---it's hard territory. > > --adrian -- -------------------------------------------------------------- Dr Jon Blower Tel: +44 118 378 5213 (direct line) Technical Director Tel: +44 118 378 8741 (ESSC) Reading e-Science Centre Fax: +44 118 378 6413 ESSC Email: [EMAIL PROTECTED] University of Reading 3 Earley Gate Reading RG6 6AL, UK -------------------------------------------------------------- ------------------------------------------------------------------------- Check out the new SourceForge.net Marketplace. It's the best place to buy or sell services for just about anything Open Source. http://sourceforge.net/services/buy/index.php _______________________________________________ Geotools-gt2-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/geotools-gt2-users
