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I am working with a dataset of the USA Marine Exclusive Economic Zone (EEZ) and a dataset of current Marine Projected Areas (MPAs). Download Data here This is all being done in ArcGIS 10.4. I know that to calculate area I must first transform the layer from GCS to PCS. This does not seem to be a problem.

enter image description here Next I calculate the area using the add geometry tool on the new PCS-transformed layer. Then I sum the area of the us EEZ I arrive at a value of ~25 Million Square Kilometers! The real size of the USA EEZ should be ~11 Million Square Kilometers.

Furthermore when I sum the area of the US MPA I arrive at a value of ~31 Million Square Kilometers. This is obviously wrong as this should be less than the total EEZ area.

Ultimately I just need to know the percent of area of the EEZ that is covered by an MPA.

Where am I going wrong with the projection or area calculation?

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First, it isn't necessary to convert the crs into a projected system, there are ways to calculate the area on the rotation spheroid.

Second, if you transform the data into a projected system. The target system should be a equal-area projection. But that's heavy for your area of interest.

The Mercator projection which you are using isn't an equal area projection.

I don't know how to calculate the areas with geographic coordinates with Esri software. But many GIS Software implementations should do this by default even if your data is projected.

Maybe you have to change some settings. In QGIS e.g. you can find the setting under Project -> Project Properties... -> General -> Measurements.

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