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# Area differences between different spatial reference systems

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07-16-2019 04:10 AM
New Contributor
Hello to all members!
I have a question regarding the spatial reference system in ArcGIS.
In order to run a hydrological model with gridded precipitation applied in a hydrological basin in Crete, Greece, I built the geometry in ArcGIS. Specifically, I projected everything in WGS 1984 UTM Zone 35N for a Hec-GeoHMS project and everything worked well.
Previously, I was using the spatial reference system: WGS 1984 World Mercator, in order to run a lumped hydrological model. I did so, because I then simulated the flood wave at the downstream, and there is compatibility with web imagery in RAS Mapper of Hec-RAS with Mercator spatial referencing.
I noticed that there are differences between the calculated area of the watershed in those projection systems.
Specifically, the study area is measured:
a\ 36 km2 using WGS 1984 World Mercator, and
b\ 24 km2 using WGS 1984 UTM Zone 35N.
Is it reasonable?
The features of both systems are the following:
WGS_1984_World_Mercator:
WKID: 3395 Authority: EPSG
Projection: Mercator
False_Easting: 0
False_Northing: 0
Central_Meridian: 0
Standard_Parallel_1: 0
Linear Unit: Meter (1.0)
Geographic Coordinate System: GCS_WGS_1984
Angular Unit: Degree (0.0174532925199433)
Prime Meridian: Greenwich (0.0)
Datum: D_WGS_1984
Spheroid: WGS_1984
Semimajor Axis: 6378137.0
Semiminor Axis: 6356752.314245179
Inverse Flattening: 298.257223563

WGS_1984_UTM_Zone_35N:
WKID: 32635 Authority: EPSG
Projection:Transverse_Mercator
False_Easting: 500000
False_Northing: 0
Central_Meridian: 27
Scale_Factor: 0.9996
Latitude_Of_Origin: 0
Linear Unit: Meter (1.0)
Geographic Coordinate System: GCS_WGS_1984
Angular Unit: Degree (0.0174532925199433)
Prime Meridian: Greenwich (0.0)
Datum: D_WGS_1984
Spheroid: WGS_1984
Semimajor Axis: 6378137.0
Semiminor Axis: 6356752.314245179
Inverse Flattening: 298.257223563
Is there a justification for area deviation because of the big differences in False Easting & Central Meridian?
2 Replies
MVP Regular Contributor

Basically, WGS 84 World Mercator is a conformal map projection, meaning it preserves shape (not area).

The UTM projection is limited to a small area with a defined maximum distortion, so the area calculation should have minimal distortion within the zone.

This is a very broad answer, and I am certain that others will expand on it.

Regards,

Jim

MVP Esteemed Contributor

Jim Cousins‌ already addressed the main issue, i.e, Mercator projections distort area, especially the further you get from the equator.

I recommend reading Tissot's indicatrix helps illustrate map projection distortion  and Tissot circles