# How does Ellipsoidal Height and Geoidal Height impact water flow? [closed]

I see this as a common theme

# Ellipsoidal height doesn't dictate water flow.

Take a shot at explaining this. I think I understand this but it sounds like it would be of use to have a question about it.

Who has the best answer as to why it doesn't?

Rather than just repeating it and linking to ESRI and other docs about it.

• I could state my understanding of this, but I'm afraid it's right and I don't want to self-answer because I think someone else will do a much better job at explaining it. – Evan Carroll Oct 25 '16 at 18:52
• There are many questions that we try to find answers to on the internet, and we aim to have many of those answers to GIS questions found here at GIS SE. However, any time we ask an open question here that seeks the "best answer" we are getting into poll territory. I think it is always better to ask about something you are having a problem with, describing what you have tried in detail, and then letting us know where you are stuck. If it is an internet search that you are stuck with then describe in detail the internet searches that you have used to try and answer your own question first. – PolyGeo Oct 31 '16 at 0:13
• Every answer we seek the best answer. – Evan Carroll Dec 27 '16 at 20:41

## 1 Answer

Earth's surface is often represented as an ellipsoid and an ellipsoid is just an oblate sphere. What controls water flow is the relief and topography of the earth under the rules of gravity (or more correctly general relativity) and hypothetically pressure. I actually think water would not flow on a perfect spheroid under gravity and I think on a perfect ellipsoid under gravity it would all flow in one direction but this is not important. Local topography / relief dictates water flow as it goes from high to low, downhill. The ellipsoid does not have peaks, valleys, ridges, channels, runs, etc that control water flow.