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derived area is calculated wrong #10360
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Author Name: Gavin Macaulay - (Gavin Macaulay -) The link in the above ticket should be: http://lists.qgis.org/pipermail/qgis-developer/2006-August/000599.html |
Author Name: Martin Dobias (@wonder-sk) Calculated area of a feature depends on the ellipsoid set in QGIS options. However also after setting ellipsoid 1866 the calculated area isn't correct. Needs further investigation. Please could you give some other examples with incorrectly calculated area? Preferably simple features - e.g. rectangle. Does it calculate area correctly for layer with unprojected coordinates? The code for calculating the area has been borrowed from GRASS, thus teoretically they should give the same results. Do you know any utility that could help to check whether the area is correct? E.g. how to do it in grass? |
Author Name: Redmine Admin (Redmine Admin) Replying to [comment:2 wonder]:
Grab a GRASS location (the database that GRASS works on): [http://grass.itc.it/sampledata/spearfish_grass60data-0.3.tar.gz]
#set the region of interest
The area of the of polygon is printed to the terminal. You can create any new vector polygon with v.digit or QGIS GRASS edit. v.digit:
QGIS GRASS edit:
Make sure the boundary starting and ending nodes are snapped. Remember that GRASS polygon ("area" in GRASS lingo) is made of a boundary and centroid. Cheers, |
Author Name: Martin Dobias (@wonder-sk) Thanks for the instructions, I'll try to do some investigation on this. |
Author Name: Gary Sherman (@g-sherman) Moved to 0.8.1 |
Author Name: maris-gis-gmail-com - (maris-gis-gmail-com -) Same applys to lenght measurements in 0.8.0 on Linux. |
Author Name: Magnus Homann (@homann) Measurements have changed a bit in trunk. I don't think it makes any difference, bu it's worth a check. I'm not GRASS-savy myself. |
Author Name: Magnus Homann (@homann) Length measurment errror should be fixed in trunk now (). |
Author Name: Magnus Homann (@homann) Downloaded the full_circle.shp and tested it in trunk . With projections off, the areas were close enough (> 10 digits). With projections on, the error was as reported (less than 0.1%) |
Author Name: Magnus Homann (@homann) OK, I looked a bit at GRASS. You can calculate the area either as planimetric or ellipsoidal. In qgis 0.9, planimetric is selected when projectin is off, else it is ellipsoidal. The ellips is taken from the global Options-> Projections dialog. If the reported value from GRASS was in fact the planimetric area, it reports the same as qgis does. (*) Why not from canvas projection ?
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Author Name: Magnus Homann (@homann)
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Author Name: Anónimo (Anónimo) Milestone Version 0.9 deleted |
Author Name: Redmine Admin (Redmine Admin)
Original Redmine Issue: 301
Redmine category:vectors
Assignee: Magnus Homann
Please refer to the thread where I'm reporting this in detail:
[http://lists.qgis.org/pipermail/qgis-developer/2006-August/000599.htm]l
Maciek
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