On Fri, Jun 13, 2008 at 9:43 AM, Jon Blower <[EMAIL PROTECTED]> wrote: > Hi Andy, > > Thanks very much for this. Yes, please pass on anything you like to > the coverages working group. > > I guess I'm not completely clear on what a Coverage actually is, > conceptually. From the GeoAPI javadocs: > > "The essential property of coverage is to be able to generate a value > for any point within its domain." > > At first read, this implies to me that a coverage is, from the point > of view of the user, a continuous feature that produces values at any > point in the domain, by interpolation if necessary (i.e. if the > coverage is stored internally as discrete elements). It worries me > that the methods in Coverage don't allow the user to specify an > interpolation method - this really matters in science. However, see > below. >
ContinuosCoverage explicitly allows to specify interpolation > However, this raises the question of what constitutes the "domain" of > a coverage. Let's assume we're dealing with the case where a coverage > is represented internally as a set of discrete points. Is the domain > simply the points themselves? Or their convex hull? Or some > arbitrary bounding volume, defined by the data provider (i.e. the > Envelope)? > The domain of a DiscreteCoverage in this case should be constituted by the collection of the single points that are the back end of the discrete coverage itself. If we then want to create a ContinuousCoverage out of this DiscreteCoverage we can define a ContinousCoverage whose domain is the convex hull (or the envelope) of the DiscreteCoverage domain (which is a collection of points) and use the interpolation between the points of the discrete coverage (in this sense a GridCoverage2D is a ContinuosCoverage that does NN itnerpolation). I have grabbed an old 19123 and the difference is subtle but seems to exist. Specifically there exists a chapter for DiscretePointCoverage which explicitly mention the fact that it could be used as a base for a ContinuosCoverage by means of interpolation. > If the domain is simply the points themselves, what would I expect to > get if I queried for a value at a different point? If I'm reading the > GeoAPI javadocs correctly then the Coverage.find() methods should > return the nearest point, whereas the evaluate() methods would throw > a PointOutsideCoverageException. However, some methods explicitly say > that this exception is thrown when the point is outside the Coverage's > Envelope, implying that the Envelope defines the domain. Is this > right? > > I guess this all boils down to the following question: is the Domain > of a Coverage always a contiguous rectangular region of space, i.e. > its Envelope (in which case interpolation is implicit)? Or can the > domain be a set of discrete points (in which case interpolation is > disallowed)? I believe the second would apply for a DiscreteCoverage backed by points. Let's just think to the case of sparse points and then you realize why you don't want interpolation in DiscreteCoverage- Simone. > > Cheers, Jon > > On Thu, Jun 12, 2008 at 10:48 PM, Andy Turner <[EMAIL PROTECTED]> wrote: >> Hi Jon et al, >> >> Jon's are all good questions, sorry I went off on a tangent. I don't know >> about any of the numbered ones, but the coverages working group of the OGC >> TC might. Do you want me to pass them on? >> >> In principal I think you can use whatever generalisation of values in the >> coverage to produce an interpolated value. However I don't know if this >> principal is reflected in the standards documentation or software >> implementations. A further thing to note is that there are generalisions >> (interpolations) of values in a single variable/coverage and those that >> involve multiple variables/coverages. For instance I might chose to guide an >> interpolation based on a combination of values from multiple coverages. >> IMHO, this is still interpolation. It is even interpolation when the value >> is going to be outwith the current range of values in the coverage. >> >> Regards, >> >> Andy >> >> >> -----Original Message----- >> From: [EMAIL PROTECTED] on behalf of Jon Blower >> Sent: Wed 11/06/2008 17:55 >> To: [EMAIL PROTECTED] >> Cc: Andy Turner; [email protected] >> Subject: Re: [Geotools-gt2-users] [GeoShaver] Re: [GeoShaver] Re: Status >> ofCoverages inGeoAPI and GeoTools >> >> Ah, interesting. Just to check I've understood this correctly: >> >> 1) A coverage represented internally by a grid (e.g. numerical model >> output, digital photo or satellite image) is a type of >> DiscreteCoverage in ISO19123 (perhaps a DiscreteGridPointCoverage)? >> >> 2) For a DiscreteCoverage, if I ask for a value at a point that >> doesn't correspond exactly with a grid point I will get the value from >> the nearest grid point (i.e. nearest-neighbour interpolation). Or do >> I actually get null? >> >> 3) However, in GeoTools, the DiscreteCoverages are not implemented. >> The GridCoverage2D class does not implement ISO19123 interfaces but it >> is effectively a discrete coverage that performs nearest-neighbour >> interpolation. >> >> 4) If I want bilinear (or bicubic or anything else) interpolation then >> I need to take my GridCoverage2D and somehow convert it to an >> Interpolator2D. How do I do this? >> >> Another question: Is it possible to have different interpolation >> methods in different directions (e.g. nearest-neighbour in the >> horizontal, linear in the vertical)? >> >> Cheers, Jon >> >> On Wed, Jun 11, 2008 at 4:20 PM, Martin Desruisseaux >> <[EMAIL PROTECTED]> wrote: >>> >>> Andy Turner a écrit : >>>> I think a continuous coverage is one where for any location a value is >>>> obtained by interpolation and that for a discrete coverage the value at >>>> any point is known directly. (The distinction, continuous or discrete is >>>> nothing to do with the values attributed to the coverage.) >>> >>> Yes it is also my understanding. It GeoTools implementation, GridCoverage2D >>> (without subclassing) would be a discrete coverage (minus tricky boundary >>> issues >>> like the ones you mentioned) while Interpolator2D would be a continuous >>> coverage. But at this time GeoTools does not implement ISO interfaces that >>> way. >>> However this is something we would like to do in the future. >>> >>> Martin >>> >>> --~--~---------~--~----~------------~-------~--~----~ >>> You received this message because you are subscribed to the Google Groups >>> "GeoShaver" group. >>> To post to this group, send email to [EMAIL PROTECTED] >>> To unsubscribe from this group, send email to [EMAIL PROTECTED] >>> For more options, visit this group at >>> http://groups.google.co.uk/group/geoshaver?hl=en-GB >>> -~----------~----~----~----~------~----~------~--~--- >>> >>> >> >> >> >> -- >> -------------------------------------------------------------- >> 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 >> >> > > > > -- > -------------------------------------------------------------- > 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 > -- ------------------------------------------------------- Eng. Simone Giannecchini President /CEO GeoSolutions S.A.S. Via Carignoni 51 55041 Camaiore (LU) Italy phone: +39 0584983027 fax: +39 0584983027 mob: +39 333 8128928 http://www.geo-solutions.it ------------------------------------------------------- ------------------------------------------------------------------------- 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
