[R-sig-Geo] overlay polygons (from shp) and raster (from geotif)

[Agustin Lobo]

Thanks,

but note that:

 > "Large cells and small
 > polygons is not really the usual case."

...except when you carry out ground work for satellite imagery!

According to the doc, starspan counts pixels intersected by the vector
provided the intersection is larger than a user-define threshold, thus 
would not solve the problem either (but have not actually tried it yet).

In R, may be I could
1. Vectorize the raster cells (only those that are not NA in delme), 
thus creating av.
2. Intersect the 2 polygons, thus creating a new set of polygons (pols2)
3. Use overlay with the new polygon pols2 and the vectorized version 
(av) of the raster a.

What do you think?

Agus


Roger Bivand escribió:
> On Tue, 23 Oct 2007, Agustin Lobo wrote:
> 
>> Hi there!
>>
>> I'm trying to do the overlay of polygons on raster.
>>
>> I do:
>>
>> pols <- readOGR("../AllTransectPolygons02", 
>> layer="AllTransectPolygons02")
>> a <- readGDAL("t1s2comb/t1s2combfim95.tif") #geotif file
>>
>> str(a) shows the correct proj info:
>>   ..@ proj4string:Formal class 'CRS' [package "sp"] with 1 slots
>>   .. .. ..@ projargs: chr " +proj=utm +zone=18 +south +ellps=WGS72
>> +units=m +no_defs"
>>
>> My goal is to get, for each polygon in pols,
>> the surface covered by each value of a.
>>
>> Then:
>>
>> delme <- overlay(pols, a)
>> delme
>>      AREA PERIMETER Site
>> NA     NA        NA <NA>
>> NA.1   NA        NA <NA>
>>
>> which is not too useful and
>>
>> delme2 <- overlay(a, pols)
>>
>> takes for ever and what I finally get is:
>>
>> > str(delme2)
>>  num [1:207024] NA NA NA NA NA NA NA NA NA NA ...
>> > delme2[!is.na(delme2)]
>>  [1]  86  86  86  86  87  87  87  87  88  88  88  88  89  89  89  89
>> 90  90
>> [19]  90  90  90  91  91  92  92  93  93  93  93  94  94  94  95  95  
>> 95  96
>> [37]  96  96  96  98 101 101 101 101 102 102 102 102 104 104 104 104 
>> 104 104
>> [55] 104 104 105 105 105 105 105 105 106 106 106 106
>>
>> that is, just the ids of the polygons on the cells of a.
> 
> This is what you would need, using this column to make a 
> SpatialPixelsDataFrame out of the input data, then doing a tapply or 
> similar to tabulate the categories.
> 
> But with 200K cells, large cells, and small polygons, the 
> point-in-polygon approach is converting the cells to their cell centre 
> points, and missing most of the cell-polygon overlaps, that is those 
> where the cell centre falls outside the polygon. Large cells and small 
> polygons is not really the usual case. In addition, it might be helpful 
> to subset pols first to avoid looping over polygons that cannot belong 
> to a.
> 
> I'm not sure how starspan does polygon/cell overlay - if it measures the 
> overlap area, you might find that more helpful. If not, reduce pols to 
> the a area, and resample a to a finer resolution so that multiple cells 
> fit inside each polygon (change g.region in GRASS, for example). It will 
> take time to do, because it does a C point-in-polygon operation for each 
> polygon (in nested lapply() calls).
> 
> Hope this helps,
> 
> Roger
> 
>>
>> Note that the extent of pols is much larger than
>> the one of a (i.e., may polygons are outside the raster and I'm not
>> interested on those). Also, cells are much larger then the width of
>> the polygons (I can send a jpg with an example).
>>
>> So it's clear I'm doing something wrong... any advice?
>>
>> Thanks
>>
>>
>>
> 

-- 
Dr. Agustin Lobo
Institut de Ciencies de la Terra "Jaume Almera" (CSIC)
LLuis Sole Sabaris s/n
08028 Barcelona
Spain
Tel. 34 934095410
Fax. 34 934110012
email: Agustin.Lobo at ija.csic.es
http://www.ija.csic.es/gt/obster