2

I used

Vector -> Research tools -> Vector grid

with enabling

Output grid as polygons

to create a 1000 by 1000 m grid from a shapefile. However, when I calculate the area of each grid cell via

attribute table -> field calculator -> Create a new field
Output field name: area
Output field type: Decimal number (real)
Output field width: 15; Precision: 5

For the Expression to calculate the area I use:

$area

When doing this, I get values which are bised by ca. 1% (too small): eg a grid cell area of 0.991639 km2 instead of 1.0 km2. The error seems to be bigger than just a rounding error and this adds up to a wrong total area of the shapefile.

  • 3
    what projection/CRS is your map in? – Ian Turton Mar 31 '16 at 8:51
  • "Layer Spatial Reference System: +proj=utm +zone=33 +datum=WGS84 +units=m +no_defs." So from that side it should be fine. – felim Mar 31 '16 at 9:41
  • Please edit the question to specify how you are calculating area (it appears to be geodetic) – Vince Mar 31 '16 at 11:43
  • edited the question according to suggestion of @Vince – felim Mar 31 '16 at 12:32
  • is each cell exactly 1 km x 1 km? – klewis Apr 5 '16 at 19:52
1

Choose a different projection. I'm not as clued up as I should be with map projections, but I seem to remember that utm has a scale attached to. I remember it being in the order of what you mentioned you were getting.

It also depends where on the globe your grid is situated and how small the area is covered by your grid.

I think Albers Equal Area projection may help for large areas. Maybe a Gauss Conformal projection for smaller areas. The shape may, however,become distorted though.

  • As you said, the problem is that creating the grid in another projection leads to distortion in the target grid in UTM (which I need since it's input to a model which requires UTM). That's why I used the follwoing workflow, however without success! - create 1000x1000 m2 grid in target projection (UTM 33). - reproject to Albers Equal Area projection for Europe (EPSG:102013); do the area calculation here - reproject back to target projection. Problem persists! Strange fact: I followed the workflow described in the question using another shapefile for an area in UTM 32, which worked just fine. – felim May 13 '16 at 7:47

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