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I don't think that this is related to #4955 because that issue is different: if the column is not wide enough the value is cut at the end (and there are no differences between the values computed by the field calculator and the "add geometry columns" tool), in this case the values are completely different.

Giovanni Manghi wrote:

After all the two issues are not that unrelated.

If you compute areas with the field calculator in a enough wide field, then the values make sense

ID Area_FC
25967 8550196769.93
26570 173647473841.47

the same values are computed by postgis.

the same values are computed by postgis.

so I would try to understand why values computed with R are different and eventually close this as duplicate.

ok, it seems there are 2 issues here:
1. The wrong results I get with
vector/Geometry Tools/Export/Add Geometry Columns
that are just missing digits to the right for large numbers

Giovanni: which version are you using?

ok to close this part as a duplicate, but the user cannot set the width of the field
in vector/Geometry Tools/Export/Add Geometry Columns, so this specific detail should be
added to #4955

2. The fact that results provided by R are so different. The most intriguing/worrying is
that the R results are using package sp and package Rgeos.
I'm dealing with Edzer Pebesma regarding this issue in R-sig-geo and will keep the list
informed. Perhaps I will open a new ticket on this specific subject but lets wait
to see what Edzer says.

Agus

alobo - wrote:

ok, it seems there are 2 issues here:
1. The wrong results I get with
vector/Geometry Tools/Export/Add Geometry Columns
that are just missing digits to the right for large numbers

Giovanni: which version are you using?

I'm using master, but the issue is definitely #4955

Update to issue 2: clarified thanks to Roger Bivand, no need to open another
ticket, QGIS, postgis, and R are coincident (but R is much faster than QGIS, few seconds
against few minutes):

"With rgeos 0.2-5 and GEOS runtime version: 3.3.2-CAPI-1.7.2, I have:

bn <- readOGR(".", "BorneoLCv2google")
b2 <- gArea(bn, byid=TRUE)

print(b2[25967], digits=16)

25966
8550196769.93012

print(b2[26570], digits=16)

26569
173647473841.3217

so the hole warnings were the reason for the difference. SVN revision 327 of 16 March enforced the use of createSPComment() in gArea() - the user was expected to denote holes properly themselves before this change. Update your rgeos and try again.

Please update the QGIS #5332 to state that from rgeos 0.2-5, the values are correct. The area slot in the Polygons object is a gross area used to order plotting (because until recently R polygons simply overplotted one another), so it is not guaranteed to be a planar area measure for polygons with holes.

Roger
Roger Bivand
Department of Economics, NHH Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: [email protected]
"

alobo - wrote:

(but R is much faster than QGIS, few seconds
against few minutes

counts made with the field calculator take also just a few moments (2/3 seconds in my machine), instead of minutes of the tool in the "vector" menu.