# Creating a custom datum transform to convert from sphere to spheroid using ArcMap?

I have Lat/Lon data that is for a sphere, but I am trying to convert these points to an oblate spheroid. Currently the sphere-based data points are offset when projected onto a map that is based on the spheroid geographic coordinate system. I believe this requires a custom datum transform, but it is not included in ArcMap and I have to specify the transform.

Any idea on how to convert latitudes used on a sphere-based geographic coordinate system to one for an oblate spheroid?

The spheroids are for Mars. One is a sphere with radius 3396190, and one is a spheroid with equatorial radius 3396190 and polar radius 3376200. It requires a custom datum transform, but I am unsure how to define such a custom transform. How is this done?

• Please Edit the question to specify which sphere, and which spheroid, and give an indication of the accuracy of the collected values, and the envelope of the collected values. – Vince Mar 26 at 18:14
• Done, thank you. It is both custom spheroids: one a sphere radius 3396190, and one a spheroid with equatorial radius 3396190 and polar radius 3376200. – ricitron Mar 26 at 23:29
• The 3396190 sphere is actually defined by the Mars 2000 datum. You'd need to use a custom datum and custom transformation to get to the other. @mkennedy is the keeper of the keys to the realm of transformations with Esri products. – Vince Mar 27 at 11:32
• Thank you @Vince , I have updated the question. Yes, I am curious how to construct a custom datum transform from a Mars sphere to a spheroid, or alternatively from the Mars2000 oblate spheroid to the Mars200 sphere. – ricitron Mar 27 at 19:55
• Is the offset entire north-south? If so, try this. In ArcMap, add both datasets. Open data frame properties, coordinate system tab, click transformations, then new transformation. Set the input/output gcs to the 2 gcs. Method to geocentric translation. Leave the parameters set to zeroes. If this works, I'll write it up as an answer. – mkennedy Mar 28 at 9:58