Area differences between different spatial reference systems

sofiasarchani — Tue, 16 Jul 2019 11:10:24 GMT

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.

Re: Area differences between different spatial reference systems

JimCousins — Tue, 16 Jul 2019 17:26:57 GMT

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

Re: Area differences between different spatial reference systems

JoshuaBixby — Tue, 16 Jul 2019 18:04:35 GMT

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

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Area differences between different spatial reference systems

Re: Area differences between different spatial reference systems

Re: Area differences between different spatial reference systems