New answers tagged area
1) Polygons to lines 2) Lines to polygons 3) Dissolve 4) Select polygons from point 2) < 200 m2 5) Difference polygons from point 3) with polygons from point 4)
All used shp-Files have the EPSG 31468. I run the model, add a field and calculate the area the sum of all records will be 10275 m² When I do the same with field calculator form the gui the sum of all records will be 10311 m². When I use a other gis I get always this result
WGS 84/NSIDC EASE Grid-Global seems to be a stereographic projection, which does not preserve areas. This is the reason why you can't get meaningful results. The most simple solution is to reproject your layer to a UTM projection suited to your area of interest.
After digitizing the polygon you know the coordinates of the vertexes from them the area can be calculated: area = sum(x(i) * x(i+1) - y(i) * y(i+1)) / 2 If you have n points and i = n then use 1 for i+1. The area may be negative depending on the order of points (clockwise or counter clockwise), use the absolute value. You can find more details here: ...
you can use an equal area projection to get the area corresponding to the surface of the Earth. Using shapely is not a problem if you use the appropriate projection. Just make sure that you densify your shapefile vertics before you project it. Examples of equal area projections include cylindrical equal area and sinusoidal projection.
If line to polygon does not work, try checking the topology and look for mistakes. Contour lines are not always closed, you could try closing them before you try the line to polygon.
basically you could reclassify your image and directly read the pixel count in the raster attribute table. Multiplying count by pixel size would give you the area. As mentioned by @whuber, this assume that you are using an equal area projection (which should also be the case if you work with vectors).
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