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Area differences between different spatial reference systems

Question asked by ssarch on Jul 16, 2019
Latest reply on Jul 16, 2019 by bixb0012
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?
Thank you in advance.

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