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so i have 'save as''d the new project and layer with my new Custom CRS project string being '+proj=laea +lat_0=70 +lon_0=0 +x_0=0 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs'.

However when I calculate area using the field calculator the numbers still don't make sense. Some of them are even in the negative. I have included a screen shot of the layer I am trying to calculate area for. Please note the column titled 'Area_Km2' are the original areas and the column titled 'Area m2' are the new areas which I am trying to calculate.

FYI I am working with a merged layer and I only clipped area in one row namely that titled 'Va2', It should be the only one with changed values. I am assuming the area will be in meters from the definition string.

enter image description here

anina
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  • Thanks. How do I know if the projection is using meters or km's in Qgis? From what I have read WG8S84 can use both, is this possible? This is the current CRS i am using http://resources.arcgis.com/en/help/main/10.1/index.html#//003r00000041000000 however I have not been able to determine if it is a PCS and not a GCS – anina Sep 19 '14 at 06:56
  • the project definition string which you can check in the CRS selection dialog contains information on the units. It's usually the last parameter of the definition string, so right at the end – underdark Sep 19 '14 at 15:46
  • Are the shapefiles you use publicly available? – AndreJ Sep 22 '14 at 11:57
  • Yes they are the ICES statistical areas available here from this website. http://geo.ices.dk/download.php?dataset=ices_ref:ices_areas. I downloaded them and split them by area, each roman numeral represents one statistical area. Note: the screen shot above represents one fish stock habitat area (a merged layer of 11 different statistical areas). – anina Sep 22 '14 at 14:22

2 Answers2

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If you are using ArcGIS Desktop, I think this link will help you with calculating area values when using data in a geographic coordintate system: http://www.gislounge.com/calculating-polygon-area-in-arcmap/

user35594
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Michael Carson
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  • In fact I am using Qgis and in this program there is no open to select units in the field calculator like there is in Arcgis. – anina Sep 19 '14 at 06:57
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I can confirm some strange results. Let's take a simple area without islands and coasts, like Va1.

The measurement tool says it is 111x155km large, so 17224 km² should be ok. It is calculated to 17099 km² by the identify tool and the field calculator.

Va2 is rather complicated, because the hole of Iceland has lots of vertices. With laea projection, I get 1900346 km², and 358115 km² with WGS84. The measurement tool calculates about 458000 km² without subtracting the hole. Wikipedia says Iceland is 103000 km² large, so the WGS84 calcualtion is ok.

If you get negative results, you might have not enough digits. To get km², divide $area by 1000000.

AndreJ
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  • Ok I have recalculated these using WGS84 and have gotten the same results as you. Even though WGS84 seems to be in lat and long with no projected units (which confuses me) the numbers are ok so It seems to have worked! I emailed ICES to ask what projection they used so that my numbers will match theirs exactly. Thanks a lot for your help on this. – anina Sep 23 '14 at 13:53
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    In case of CRS with degrees, QGIS automatically calculates the areas and distances on the ellipsoid. – AndreJ Sep 23 '14 at 14:32
  • So ICES are not sure what they used exactly. They said both WGS84 and some kind of LAMBERT. I can only seem to get recognizable figures if I set the project and layer CRS to WGS84. Any other CRS I am using produces false figures. Do you think this it is reasonable for me to use WGS84 form a scientific point of view? – anina Sep 24 '14 at 08:03
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    As long as QGIS stumbles over the coastlines in laea projection, you have no other choice. – AndreJ Sep 24 '14 at 12:20
  • Thanks. just so i can defend this decision if i need to. What do you mean by stumbling over coastlines? – anina Sep 24 '14 at 14:07
  • As I stated in my answer, a simple rectangle gets calculated ok, whike the zone with Iceland inside gets calculated wrong in laea projection. I still don't know where the bug is. – AndreJ Sep 24 '14 at 16:56
  • I see thanks a lot I will use WGS84 as my projection from now on. – anina Sep 25 '14 at 10:21