# Is NEMO considering any specific GEOID?

**URL:** <https://nemo-ocean.discourse.group/t/is-nemo-considering-any-specific-geoid/663>\
**Category:** Uncategorized\
**Created:** [8 April 2024 08:17 UTC](https://nemo-ocean.discourse.group/t/is-nemo-considering-any-specific-geoid/663 "2024-04-08T08:17:35Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Kat](https://avatars.discourse-cdn.com/v4/letter/k/b19c9b/32.png) [@Kat](https://nemo-ocean.discourse.group/u/Kat)\
**Post date:** [8 April 2024 08:17 UTC](https://nemo-ocean.discourse.group/t/is-nemo-considering-any-specific-geoid/663/1 "2024-04-08T08:17:35Z")

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Hello everyone,

I am trying to figure out if NEMO uses any specific geoid. I read in the manual that both the sea surface height and the depth of the ocean bottom are referenced to a surface of constant geopotential (i.e. a mean sea surface height) on which z = 0. So this level of reference does not consider any geoid…it seems a bit arbitrary?

Also, when constructing NEMO bathymetry, again no geoid is used? I saw in the manual that one option for the calculation of the bathymetry, is using ETOPO datasets. And these datasets as far as I saw, consider the EGM2008 geoid surface and they then convert it in WGS84 ellipsoid elevation heights. Can someone confirm that indeed this option is used for calculating NEMO bathymetry?

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**Author:** ![smasson](https://avatars.discourse-cdn.com/v4/letter/s/eb9ed0/32.png) [@smasson](https://nemo-ocean.discourse.group/u/smasson)\
**Post date:** [8 April 2024 12:49 UTC](https://nemo-ocean.discourse.group/t/is-nemo-considering-any-specific-geoid/663/2 "2024-04-08T12:49:54Z")

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In NEMO, the earth is defined as a perfect sphere with a radius defined in `src/OCE/DOM/phycst.F90`:

```fortran
   REAL(wp), PUBLIC :: ra = 6371229._wp !: earth radius [m]

```

Earth gravity is constant and also defined in `src/OCE/DOM/phycst.F90`:

```fortran
   REAL(wp), PUBLIC :: grav = 9.80665_wp !: gravity [m/s2]

```

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<div class="post-metadata">

**Author:** ![Kat](https://avatars.discourse-cdn.com/v4/letter/k/b19c9b/32.png) [@Kat](https://nemo-ocean.discourse.group/u/Kat)\
**Post date:** [9 April 2024 09:52 UTC](https://nemo-ocean.discourse.group/t/is-nemo-considering-any-specific-geoid/663/3 "2024-04-09T09:52:07Z")

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Thank you for your reply. So, this surface of constant geopotential where z = 0 and is the level of reference for NEMO is the perfect sphere?