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When a polygon shapefile is converted into a raster, an attribute from the shapefile's attribute table must be chosen for the cell value of the raster.

I am pretty sure the above statement is true, but I cannot explain why.

Can anyone explain why this is the case?

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  • It depends on what software you're working with. ArcGIS expects a field to be nominated, if you want a single burn value then add a field and calculate it to the burn value; if you are using GDAL, through QGIS or from CMD, then a single value or field can be specified. The reason why a field is nominated is because the assumption is that you would normally want to identify which polygon(s) gave a value to each cell. BTW the term burn value is a term that comes from GDAL, you will not find this term in the Esri help.
    – Michael Stimson
    Oct 21 '19 at 23:44
  • Thanks for your reply, but sorry, I don't really get your answer. I'm only referring to ArcGIS, by the way. So, this means that every raster cell HAS TO carry a value and these are taken from the attribute table of the polygon shapefile?
    – Mike
    Oct 21 '19 at 23:53
  • Yes, rasters are massive arrays of values Y by X, each address in the array must have a value but it can be NoData as you will find with the tool, should the polygons not completely cover the extent of your raster any cell that is not within a polygon will have the value of NoData. There are very limited exceptions to this but for the sake of simplicity you can use this generalization for any of the common raster formats.
    – Michael Stimson
    Oct 22 '19 at 0:03
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No, it's not true, in general. You could simply burn the features into a raster where cells are given an automatically-incrementing ID that relate to features (where this isn't a property of the features), or indeed nothing at all (e.g. a binary presence/absence surface), or using the vector dataset's extent to define a raster's extent without considering any feature-level attributes, or masking a raster based on vector features.

If this relates to the use of a particular GUI tool, e.g. some QGIS tool, then it might be true.

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