In GPS land surveying you determine the position horizontally and vertically. To determine the position horizontally you only need the latitude and longitude given by GPS. To determine the position vertically you need the altitude given by GPS and you need a geoid. So then there is always the question: “What is geoid?”. When you know the answer on this then you will know better about GPS land surveying. So here we will give you the answer on: “What is geoid?”.

## The definition of geoid tells a lot on the answer on the question ``What is geoid?``

When it is about the definition of geoid then we are lucky. There is only one definition on this word. According to the dictionary it means: a hypothetical solid figure whose surface corresponds to mean sea level and its imagined extension under (or over) land areas.

This definition only does not explain how geoids are used in land surveying and what purpose they have.

## Lets dig deeper on the geoid

When we simplify the definition of geoid then we get that a it is a shape that the ocean surface would take under the influence of the gravity and rotation of earth alone, if other influences such as winds and tides were absent.

That is very interesting. The gravity on earth is not everywhere the same. The reason for this is that the mass of earth is unevenly distributed.

## So what does it matter that gravity is not everywhere the same?

On this page there is an image of geoid EGM96. On this image you can see the differences on the strength of gravity.

We all learnt in school that water flows downwards. Actually it does not. Water goes to the place where it is more attracted by gravity.

## Geoid and GPS land surveying

When you are GPS land surveying you get information about your position. You get this position in latitude, longitude and an offset from the ellipsoid.

Heights are usually measured from a water level. That is the reason why we need the geoid. The offset from the ellipsoid given by the GPS receiver has to be modified because of the differences in gravity. So the geoid is very important to determine the orthometric height.

## GPS land surveying the easy way

That there is a gradation in gravity on earth makes GPS land surveying complicated.

But there is an easy way. We developed for you the easiest land survey app in the field. It is called Apglos Survey Wizard.

When you use this measurement app then you don’t have to worry about geoids. All the calculation of the real height is integrated in the software.