Recalculating shape area with projected coordinate system

[SilvieDaniels]

Dear,

I currently work with ArcGIS 9.3 desktop and have two database layers, have overlayed them using the intersect tool and needed to calculate the new shape areas but have encountered some problems to which I cannot find a solution 🙂

The first dataset I am working with is one with the European regions (NUTS3), the second dataset shows the different variables for soil parameters in Europe and wanted to link both files and know which European region has which soil parameters. I used the tool intersect. Since there are many different soil parameters, the original European regions have now been subdivided into several smaller regions. As a test I summed up the shape areas of the smaller regions, which would need to be equal as the larger European region. However this was not the case. I tried using the calculate geometry tool and ArcGIS mentioned that I needed to use a Projected Coordinate System (PCS) instead of a Geographic Coordinate System (GCS) in order to perform the calculate geometry.

So, I adapted the coordinate system from WGS to UTM (Layer properties>Coordinate system>predefined>projected coordinate system>UTM) However, there does not seems to be one projection system for the whole of Europe, as the UTM zones are determined for smaller regions and not for Europe as a whole? This is I think the first problem.

Then, when I chose a UTM zone (WGS 1984 UTM zone 31S for example) ArcGIS did calculate the shape areas for the whole of Europe (again using the calculate geometry tool) but again the sum of the smaller regions did not sum up to the total area of the whole region.

Does anyone know what mistake I am making?

[FabianBlau]

Check if your 2 layers have the same coordinate system.
Check also the coordinate system of the dataframe.
Is the correct transformation choosen?

[SilvieDaniels]

The two original files were both in the same geographic coordinate system: GCS_WGS_1984. I then used the intersect tool and am now working with the file combining both layers. So I did not do a tranformation before adding the two layers together. Since the calculate geometry tool did not work on geographic coordinate systems, I have tried to transform the intersected layer to a projected coordinate system but there did not seem to exist one PCS for the whole of Europe?

[FabianBlau]

The area depends on the projection  (http://en.wikipedia.org/wiki/Map_projection). For your comparison it should not so important, take any UTM-Projection (but dont use the areas for other reasons!).
Your regions-layer should be in the same projection. A different projections means usually other areas.
How do you know the areas of the regions?

[SilvieDaniels]

Thank you Fabian. Just to see that I understand you correctly:

LAYER 1 (All European regions) : According to the data file description this layer is in ETRS 1989. However, when I checked this in Arcmap, the coordinate system for this layer was in WGS_GSC
LAYER 2 (soil data):also in WGS_GCS.
I then performed the intersect
LAYER 1 & LAYER 2 combined: transformed this layer to UTM.

1. Is it ok that both layers before the intersect are in WGS (although the data file manual says it is in ETRS)?
2. Or do you mean that before the intersect, that both layers need to be in the UTM projection instead of the WGS?

Then, regarding the area calculation:
LAYER 1: The European regions layer contained all the surface areas
LAYER 2:The soil layer did not.
LAYER 1 & LAYER 2 combined: After the intersect, the regions become divided into smaller regions and surface areas are calculated for all the smaller regions. However, these do not sum up to the the surface area of the whole region as provided by layer 1.

1. My colleague told me that it wasn't necessary to change to a UTM projection to calculate the regions surface area, what is your experience?
2. If my projections would have been correct, would I still need to recalculate geometry or should the calculation after the intersect be correct automatically?

If I understand your question correctly, the regions in the data layer are all European regions and are also showing on the map.
Thanks

[FabianBlau]

1. Is it ok that both layers before the intersect are in WGS (although the data file manual says it is in ETRS)?

I dont know.

2. Or do you mean that before the intersect, that both layers need to be in the UTM projection instead of the WGS?

To compare LAYER 1 (All European regions) and your result, first reproject LAYER 1 to UTM and recalculate the areas in Layer 1.
Compare the new areas with your intersection-results.
Areas depends on the projection. Comparing areas only make sense if the underlying data have the same projection.

LAYER 1: The European regions layer contained all the surface areas

Try to find out the source of the areas. Are there any informations in the data file description?

1. My colleague told me that it wasn't necessary to change to a UTM projection to calculate the regions surface area, what is your experience?

http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/Calculating_area_length_and_other_geom...

2. If my projections would have been correct, would I still need to recalculate geometry or should the calculation after the intersect be correct automatically?

I dont know. Try it: Add a new field, calulate the areas and compare.

[SilvieDaniels]

Thank you!

I first reprojected both layers to Projection Coordinate System ETRS 1989. After performing the intersect I recalculate the areas using calculate geometry and now they do add up. The key hence was reprojecting before intersecting 🙂