I have a large raster dataset within a file geodatabase (FGDBR format raster) which I have converted to polygon using the 'Raster to Polygon' tool.

Cell size of original raster = 25m.

Cell size of polygon = 25m.

Total area of original raster = 10,423,764 ha.

Total Area of polygon dataset = 10,235,534 ha.

Total area of raster when values extracted then added together (see 5th paragraph) = 10,235,534 ha.

Projected coordinate system = British National Grid

The area of the resulting polygon dataset is 1.8% less than area the cells of the original raster. However, visually; there is no difference between the two datasets (checked by overlaying them in ArcMap), except when 'zoomed' right out, then you can see slithers of the original raster cells outside the polygons. When scale is less than 1:200,000, the slithers disappear.

I calculated the original raster area by multiplying the cell count by the area of each square. I did not check the 'Simplify Polygons' option when using the tool.

I've also created the polygon dataset in different ways, such as using the 'Extract by Attributes' tool to extract separate parts of the raster and converting each part to polygon. But this also results in an area of 1.8% less, but no visual difference except at 1:200,000 scale or more.

I've also used Raster Calculator to subtract the extracted parts of the raster from the original raster, but this results on a raster with a value of 0 throughout.

Can anyone explain what is going on here please?

  • you should add more details about your original raster dataset. what cell sizes are you using? what's the total area covered by your raster? what coordinate system are you using? – dru87 Sep 14 '17 at 14:17
  • I have added these now – ffinnm Sep 14 '17 at 14:24
  • 1
    Snap raster and extent are to be equal to source. – FelixIP Sep 14 '17 at 20:14
  • Hi @FelixIP can you expand on that please. How do I do this? – ffinnm Sep 15 '17 at 7:39
  • 1
    Go to environment settings and set 3 things, extent, snap, cell size all pointing to raster – FelixIP Sep 15 '17 at 7:49

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