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From: Milton Jonathan <milton@te...>  20100713 15:41:30

Hello We have a simple issue down here: we want to compute the area of an arbitrary polygonal feature. The straightforward way I thought about was to simply convert that geometry to an equalarea projection, and then let JTS Geometry.getArea do the rest. However... I was surprised that I couldn't find a single equalarea projection in GeoTools that is available globally (say the user wants to compute the area of Russia, or of Antarctica). From what I saw, something like the Mollweide or Sinusoidal projections would do the job, but they don't seem to be currently implemented in GeoTools. Going around the documentation, email lists, etc. my impression is that the only available equalarea projection is the Albers Conic EqualArea. However, it's not clear to me how I should play around with the standard parallel parameters (ok, would need to study it a little), but more than the that I'm really not sure how this would behave for features that cross the equator. Any help appreciated! Cheers Milton  Milton Jonathan Grupo GIS e Meio Ambiente Tecgraf/PUCRio Tel: +552135272502 
From: Jody Garnett <jody.garnett@gm...>  20100714 14:19:19

Hi Milton; A month or so ago we went over how to generate an equal area projection (either generate the WKT using Albers Conic; or do something similar programatically). There are a number of "AUTO" projections defined in GeoTools (they take the form "AUTO:<CODE>,<lat>,<lon>"). But I am not aware if any of them are equal area ... perhaps you could look. People do donate additional projections; perhaps you would be interested in donating one of the ones you mention? The documentation and the use of BC Albers in examples is mostly my fault as that is where I was working. Let us know how you go; I think an "AUTO" projection that used Albers Conic would be great. Jody On 14/07/2010, at 1:41 AM, Milton Jonathan wrote: > Hello > > We have a simple issue down here: we want to compute the area of an > arbitrary polygonal feature. > > The straightforward way I thought about was to simply convert that > geometry to an equalarea projection, and then let JTS Geometry.getArea > do the rest. > > However... I was surprised that I couldn't find a single equalarea > projection in GeoTools that is available globally (say the user wants to > compute the area of Russia, or of Antarctica). From what I saw, > something like the Mollweide or Sinusoidal projections would do the job, > but they don't seem to be currently implemented in GeoTools. > > Going around the documentation, email lists, etc. my impression is that > the only available equalarea projection is the Albers Conic EqualArea. > However, it's not clear to me how I should play around with the standard > parallel parameters (ok, would need to study it a little), but more than > the that I'm really not sure how this would behave for features that > cross the equator. > > Any help appreciated! > > Cheers > Milton > >  > > Milton Jonathan > Grupo GIS e Meio Ambiente > Tecgraf/PUCRio > Tel: +552135272502 > >  > This SF.net email is sponsored by Sprint > What will you do first with EVO, the first 4G phone? > Visit sprint.com/first  http://p.sf.net/sfu/sprintcomfirst > _______________________________________________ > Geotoolsgt2users mailing list > Geotoolsgt2users@... > https://lists.sourceforge.net/lists/listinfo/geotoolsgt2users 
From: Ian Turton <ijturton@gm...>  20100714 14:29:21

On Wed, Jul 14, 2010 at 10:18 AM, Jody Garnett <jody.garnett@...> wrote: > Hi Milton; > > A month or so ago we went over how to generate an equal area projection (either generate the WKT using Albers Conic; or do something similar programatically). There are a number of "AUTO" projections defined in GeoTools (they take the form "AUTO:<CODE>,<lat>,<lon>"). But I am not aware if any of them are equal area ... perhaps you could look. > > People do donate additional projections; perhaps you would be interested in donating one of the ones you mention? > The documentation and the use of BC Albers in examples is mostly my fault as that is where I was working. > This series of posts by Tyler may help  http://www.geowebguru.com/articles/201choosingamapprojectionpart1 Ian  Ian Turton 
From: Milton Jonathan <milton@te...>  20100714 19:13:43

Hello there Jody Unfortunately, I don't really have resources right now to study and implement complex projection computations, if that's what you meant.. :P Anyway, what I was really after was a confirmation (I guess we could say that it is confirmed) that there is no good globally valid equal area projection available in GeoTools. Given what you guys discuss here (http://www.mailarchive.com/geotoolsdevel@.../msg20717.html) no support for Mollweide exists or will be implemented in the shortterm. I am assuming that support for Sinusoidal goes the same way. I am just a little surprised because it seems that OpenLayers does support Mollweide, Sinusoidal, etc, because they are all available in proj4js. See this link: http://www.equalareamaps.com/ (by the way, proj4js seems to consider Mollweide to be EPSG:54009 and Sinusoidal to be EPSG:54008, but they are not present in GeoTools' EPSG library) On the other hand, my issue right here is just to *compute the area* of a feature; I don't need to render it. So if the Albers Conical Equal Area projection holds its "equal area" properties for the entire world, that's fine for me. Do you know if that's true?? I actually did a little test using a shapefile with Brazil's states, which includes a computed area for each state as an attribute (although I have no idea where those computed areas came from). I used the CanadaBC parameters (EPSG:3005) and then the Florida parameters (EPSG:3086), and the areas computed were almost identical and both pretty reasonable (less than 0.4% of error compared to the precomputed values). I still need to see if it gets better if I adjust the parameters for each state. I also still do not know if I could use this to compute the area of polar regions (e.g., Antarctica) Cheers Milton Jody Garnett wrote: > Hi Milton; > > A month or so ago we went over how to generate an equal area projection (either generate the WKT using Albers Conic; or do something similar programatically). There are a number of "AUTO" projections defined in GeoTools (they take the form "AUTO:<CODE>,<lat>,<lon>"). But I am not aware if any of them are equal area ... perhaps you could look. > > People do donate additional projections; perhaps you would be interested in donating one of the ones you mention? > The documentation and the use of BC Albers in examples is mostly my fault as that is where I was working. > > Let us know how you go; I think an "AUTO" projection that used Albers Conic would be great. > > Jody > > On 14/07/2010, at 1:41 AM, Milton Jonathan wrote: > >> Hello >> >> We have a simple issue down here: we want to compute the area of an >> arbitrary polygonal feature. >> >> The straightforward way I thought about was to simply convert that >> geometry to an equalarea projection, and then let JTS Geometry.getArea >> do the rest. >> >> However... I was surprised that I couldn't find a single equalarea >> projection in GeoTools that is available globally (say the user wants to >> compute the area of Russia, or of Antarctica). From what I saw, >> something like the Mollweide or Sinusoidal projections would do the job, >> but they don't seem to be currently implemented in GeoTools. >> >> Going around the documentation, email lists, etc. my impression is that >> the only available equalarea projection is the Albers Conic EqualArea. >> However, it's not clear to me how I should play around with the standard >> parallel parameters (ok, would need to study it a little), but more than >> the that I'm really not sure how this would behave for features that >> cross the equator. >> >> Any help appreciated! >> >> Cheers >> Milton >> >>  >> >> Milton Jonathan >> Grupo GIS e Meio Ambiente >> Tecgraf/PUCRio >> Tel: +552135272502 >> >>  >> This SF.net email is sponsored by Sprint >> What will you do first with EVO, the first 4G phone? >> Visit sprint.com/first  http://p.sf.net/sfu/sprintcomfirst >> _______________________________________________ >> Geotoolsgt2users mailing list >> Geotoolsgt2users@... >> https://lists.sourceforge.net/lists/listinfo/geotoolsgt2users > >  Milton Jonathan Grupo GIS e Meio Ambiente Tecgraf/PUCRio Tel: +552135272502 
From: Andrea Aime <aaime@op...>  20100714 19:36:06

Milton Jonathan ha scritto: > Hello there Jody > > Unfortunately, I don't really have resources right now to study and > implement complex projection computations, if that's what you meant.. :P > > Anyway, what I was really after was a confirmation (I guess we could say > that it is confirmed) that there is no good globally valid equal area > projection available in GeoTools. Given what you guys discuss here > (http://www.mailarchive.com/geotoolsdevel@.../msg20717.html) > no support for Mollweide exists or will be implemented in the > shortterm. I am assuming that support for Sinusoidal goes the same way. Hi Milton, if libproj has the code to support a certain projection it's normally quite easy to port it over to GeoTools. Last time I tried I could port over at least one projections a day, tests included. It's just that I have currently no reason to try and support more projections and I'm also pretty busy these days. Here is a small guide on how to add a new projection starting from proj4 code: http://docs.codehaus.org/display/GEOTDOC/How+to+add+new+projections Cheers Andrea  Andrea Aime OpenGeo  http://opengeo.org Expert service straight from the developers. 
From: Milton Jonathan <milton@te...>  20100716 15:38:00

Hello there Good to know that Andrea. Anyway, in my case here, available Albers projections (e.g., EPSG:3005) worked fine for Brazil. If I ever notice a deficiency and need another projection that is not available, I now know that it won't be anything otherworldly to implement :) Thanks Milton Andrea Aime wrote: > Milton Jonathan ha scritto: >> Hello there Jody >> >> Unfortunately, I don't really have resources right now to study and >> implement complex projection computations, if that's what you meant.. :P >> >> Anyway, what I was really after was a confirmation (I guess we could >> say that it is confirmed) that there is no good globally valid equal >> area projection available in GeoTools. Given what you guys discuss >> here >> (http://www.mailarchive.com/geotoolsdevel@.../msg20717.html) >> no support for Mollweide exists or will be implemented in the >> shortterm. I am assuming that support for Sinusoidal goes the same way. > > Hi Milton, > if libproj has the code to support a certain projection it's normally > quite easy to port it over to GeoTools. > Last time I tried I could port over at least one projections a day, > tests included. > > It's just that I have currently no reason to try and support more > projections and I'm also pretty busy these days. > > Here is a small guide on how to add a new projection starting > from proj4 code: > http://docs.codehaus.org/display/GEOTDOC/How+to+add+new+projections > > Cheers > Andrea > >  Milton Jonathan Grupo GIS e Meio Ambiente Tecgraf/PUCRio Tel: +552135272502 
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