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I'm trying to map medieval field systems in QGIS, this means that I'll be drawing quite a few polygons. I've mapped quite a few fields already using Spatialite layers using polygons, though I was wondering if using multipolygon would be better?

Ultimately I want to calculate the Acreage of these fields, something which I have found to be very difficult at the moment in QGIS.

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  • If you know that all your polygons are single part ones with only one outer shell you can use either polygons or multipolygons. If you have now or in the future at least one real multipolygon with two or more outer shells then you must use multipolygons.
    – user30184
    Commented Jan 8, 2018 at 12:10
  • Hi I've added an image of what I've been mapping, these are all separate polygons
    – BoloBoffin
    Commented Jan 8, 2018 at 12:54
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    I cannot help you with the decision of polygons or multipolygons, but I wonder why you find difficult to calculate the acreage? Isn't it the area of each plot?if yes, have a look to the question gis.stackexchange.com/questions/23355/… and the operator $area
    – Marco
    Commented Jan 8, 2018 at 13:56
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    You have an issue with the CRS of your project. We can keep the way of calculating the area for later, because when I see that you have coordinates like UTM in meters, a scale of 220 millions and your project has as CRS the EPSG 4326, my guess is that you are very new with GIS and you are using the wrong CRS for your data without noticing. You will not get too much attention under this question which is a kind of advanced decision about how to handle gis data. I suggest you to start another queston about how to set up a project for your main porpose
    – Marco
    Commented Jan 8, 2018 at 20:42
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    BTW, I think for your purpose, polygons is the right approach
    – Marco
    Commented Jan 8, 2018 at 20:55

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The decision between multipolygons and polygons is really up to you. The only difference is that you can have one FEATURE with multiple GEOMETRIES if you use a multi- type.

So, if you had a field that was split by a road, say, but it had the same name or ID, then you might want to use a multipolygon to represent that field as a single feature. If you used a polygon type, you would need to create two features, one on each side of the road.

Calculating acreage should be easy. I achieved this in the screenshot below by using the field calculator in the attribute table, using the formula $area * 0.000247105 (because my projection is in metres, and to go from metres to acres, you multiply by that number).

Calculation of acres

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