QGIS » QGIS Application

The link in the above ticket should be:

http://lists.qgis.org/pipermail/qgis-developer/2006-August/000599.html

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?

Replying to [comment:2 wonder]:

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?

Grab a GRASS location (the database that GRASS works on): [http://grass.itc.it/sampledata/spearfish_grass60data-0.3.tar.gz]

1. install GRASS; you don't need the latest and gratest - anything above 6.0 will do

2. run GRASS

3. for DATABASE, enter the path to the directory where you unpacked the spearfish location; put "spearfish60" for LOCATION and "PERMANENT" for MAPSET

1. spawn a display
   5. d.mon x0
2. display vector
   6. d.vect your_vector_map
3. query it
   7. d.what.vect -x ; l-click the polygon

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:

1. v.digit -n map_name
2. (GUI pops up) draw a boundary and a single centroid inside
3. exit v.digit; the vector is written - display it, query

QGIS GRASS edit:
1. GRASS plugin > Open mapset
2. GRASS plugin > Create new GRASS vector
3. draw a boundary and centroid inside

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,
Maciek

Thanks for the instructions, I'll try to do some investigation on this.

Moved to 0.8.1

Same applys to lenght measurements in 0.8.0 on Linux.
I loaded Spearfish roads as shapefile (from GRASS export). Projection was set automaticly to one exported from GRASS (looked OK). Measured single road length with "Measure length" tool. Looked OK (~860m), but "Identify" tool shoved only 0.040 km.

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.

Length measurment errror should be fixed in trunk now ().

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%)

With df2b8b28 (SVN r6535), planimetric measurements are available as an option (for 0.9)

Milestone Version 0.9 deleted