[postgis-devel] Status of raster mapalgebra?

[Jorge Arévalo]

On Mon, Jan 31, 2011 at 10:15 PM, Nicklas Avén
<nicklas.aven at jordogskog.no> wrote:
> Thanks Pierre
>
> I will take a look at the plpgsql-functions.
>
> I understand that there is a lot of work to do. It will be exciting when
> things are in place :-)
>
> /Nicklas
>

Hi,

1-raster version of MapAlgebra should be implemented as core function
(C function) for the end of this week. I'll keep you informed anyway.
There's a huge amount of work to do, and not as much time as I'd like,
but it's an exciting work :-)

Best regards,

-- 
Jorge Arévalo
Internet & Mobilty Division, DEIMOS
jorge.arevalo at deimos-space.com
http://es.linkedin.com/in/jorgearevalo80
http://mobility.grupodeimos.com/
http://gis4free.wordpress.com
http://geohash.org/ezjqgrgzz0g

>
>
> On Mon, 2011-01-31 at 15:20 -0500, Pierre Racine wrote:
>> Hi Nicklas,
>>
>> >What is the status of map algebra functionality?
>>
>> There is a one raster version on the way (Jorge what is the status?) and a two raster version planned. You can see slow plpgsql prototypes of those two functions in raster/script/plpgsql
>>
>> >Is there any code that could be tested?
>>
>> Just the plpgsql versions. But they might be sufficient to make a demo or proof of concept.
>>
>> >In beginning of Mars I will give a talk about PostGIS i Stockholm. I
>> >will try to tell about the news in 2.0.
>>
>> MapAlgebra will be a very progressive work:
>>
>> 1) One raster version - Pixel by pixel
>> 2) One raster version - Accepting neighbour pixels in expression
>> 3) One raster version - Accepting neighbour pixels from the whole coverage (hence getting raster values using world coordinates over the whole table instead of just raster coordinates)
>> 4) Two raster version
>> 5) Two raster version working with non-aligned rasters (forcing resampling of one to be aligned on the other one)
>>
>> 1) might do it in 2.0.
>> 2) maybe.
>> The rest probably/certainly not. It will depend on the date of release.
>> 5) need GDAL based ST_Resampling first (this is still far away).
>>
>> >Most of my raster experience is about finding clear cuttings by
>> >comparing satellite imagery from different years. It would have been
>> >very nice to be able to show that in PostGIS. The software that were
>> >used for that purpose was something called Enforma that as far as I
>> >understand just compared band for band what had happened since last
>> >year. The different bands give different result depending on what
>> >wave-lengths the photosynthesis uses compared to what is absorbed to
>> >naked ground and so on.
>>
>> For this I guess you would need the two raster version computing difference between two bands years after years.
>>
>> >Will there be anything to try during February?
>>
>> I suggest you use the plpgsql versions as a proof of concept and comment on their usage. They might however be broken right now because of some change in the API but they should be easy to repair.
>>
>> >Am I right that map algebra will be the tool to use?
>>
>> Yes. Something like:
>>
>> SELECT ST_MapAlgebra(rast1, 1, rast1, 2, "rast2 - rast1") FROM yourrastertable WHERE ST_Intersect(rast1, rast2)
>>
>> Later MapAlgebra should be encapsulated by a number of specific functions. In your case probably something like ST_Difference(raster1, band1, raster2, band2)
>>
>> >I have no idea how much work there is to do so I don't know what I am
>> >asking for.
>>
>> The two raster version is very complicated (you must deal with nodata values, non-aligned rasters). I'm working on an optimized version (setting large zones or raster with memcpy when possible) which still a lot of work.
>>
>> Pierre
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