# How to define Pix4D outputs with respect to a Geoid model

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Video: Watch the instructional video related to Altitude and Vertical Coordinate System: here.

The Z coordinate (altitude) can be given with respect to an ellipsoid (ellipsoidal height)  or with respect to a geoid (orthometric or geoid height). Both geoid and ellipsoid are mathematically defined surfaces; the ellipsoid (left) is relatively simple, whereas the geoid (right) is much more complex:

Note: Nowadays, Global Navigation Satellite System (GNSS), a satellite-based technology, is widely used for surveying throughout the world, and it provides the altitude with respect to the WGS84 ellipsoid, that is to say, ellipsoidal height (not orthometric).

In order to convert from an ellipsoidal height into geoid or orthometric height, there are three different approaches that could be used depending on the required accuracy:

• Constant height shift

Recommended for small areas where geoid and ellipsoid can be considered parallel in a small portion of terrain. In case of a large area or significant variations between the ellipsoid and geoid, this approach will introduce inaccuracies.

In order to define the height shift in Pix4D, the Geoid Height Above the Ellipsoid function can be used:

More information can be found here: When to use the Geoid Height Above the Ellipsoid function?

• Global geoid model
Over the years, international organizations have developed geoid models which cover the whole earth's surface. The expected accuracy is around 2-3 meters depending on the area and the geoid model. Pix4D supports the EGM84, EGM96, and EGM2008 global models:

• Local geoid model
In order to improve the accuracy of the global models, some countries established local geoid models with a better resolution to cover their territory. Typically, the local geoid models in different formats (ASCII, .grd, .tiff, etc.) and the transformation tools are provided by the national mapping institutions. Using a local geoid model will provide the best accuracy reaching up to a few cms.

Important:  For now, Pix4D does not support the use of local geoid models. Only a global geoid model or a constant shift can be applied. Therefore, whenever ellipsoidal coordinates are given, the recommendation is to convert them into orthometric by using a precise local geoid model in a third-party application.

## How does it work within Pix4D?

There are four different cases.

Note: The coordinates imported in Pix4D can be:
• Image Geolocation:
• Standard image geolocation. Most of the drones in the market provide image geolocation which has an accuracy of a few meters.
• Precise image geolocation (RTK/PPK drones). They can reach up to a few cms accuracy.
• GCPs (Ground Control Points). They are usually measured with high accuracy (2-5 cm)
• Case 1. Standard image geolocation and no GCPs
The accuracy of the project will be the same as the image geolocation (a few meters). Pix4D allows applying a constant shift or selecting a global geoid for the output vertical coordinate system. If that is applied, the result will be given with respect to the geoid, but as the image geolocation absolute accuracy is not very high, the absolute accuracy will not be high either.
• Case 2. Standard image geolocation and GCPs
When using GCPs, Pix4D software will use them for georeferencing the project. Therefore if they are already given in orthometric (geoid) height, the result will also be given in orthometric height even if the image geolocation is given with respect to the ellipsoid.

As the GCPs are usually measured with a GPS device, the measured height will be ellipsoidal. The recommendation is to convert the heights before they are imported in Pix4D by using a precise local geoid model. Some GPS devices already have the geoid models preloaded, and the transformation to the orthometric (geoid) heights is done automatically.

If the local geoid model is not available, the GCPs can be imported into Pix4D, and a constant shift or global geoid should be applied. If this is the case, the result in Z will not be precise as any of these conversions are not precise either, so the expected accuracy will be similar to the previous case.

• Case 3. Precise image geolocation and GCPs
The recommendation is to convert the heights of the image geolocation and GCPs before importing them into Pix4D by using a precise local geoid model. A constant height and a global geoid could also be used, although the accuracy would be lower.
• Case 4. Precise image geolocation and no GCPs
As there are no GCPs, the only way to get the result with respect to the geoid is by converting the image geolocation altitude beforehand using a precise local geoid model. A constant height and a global geoid could also be used, although the accuracy would be lower.

 What to do to get the result wrt the geoid Orthometric height absolute accuracy 1. Standard image geolocation and no GCPs Convert image geolocation by: a) applying a constant shift or b) using a global geoid model Same as image geolocation (a few meters) 2. Standard image geolocation and GCPs Convert GCPs by: a) applying a local geoid model beforehand or b) applying a constant shift or c) using a global model a) same as GCP accuracy (2-3 cms usually) b) (*) c) 2-3 m 3. Precise image geolocation and GCPs Convert GCPs and image geolocation (optionally) by: a) applying a local geoid model beforehand or b) applying a constant shift or c) using a global model a) same as GCP accuracy (2-3 cms usually) b) (*) c) 2-3 m 4. Precise image geolocation and no GCPs Convert image geolocation by: a) applying a local geoid model beforehand or b) applying a constant shift or c) using a global model a) same as image geolocation accuracy (2-10 cms usually) b) (*) c) 2-3 m

(*) Depends on how large the project is and where is located

This article refers to vertical coordinate systems. It is also recommended to read the following Community post about 2D coordinate systems transformations: Horizontal grid corrections and transformations.

1 out of 1 found this helpful

### Article feedback (for troubleshooting, post here instead)

• Miguel Benguría arana

Yo estoy en el caso de tener RTK, concretamente el Panthom 4 rtk, y tengo el problema de que no está cargado el geoide de  donde llevo a cabo mis trabajos. Creo que es algo obvio y necesario que Pix4D deje la opción de poder cargar el geoide local. No poner el geoide de cada parte del mundo pero sí que uno mismo pueda cargarlo. Si no no veo muy práctico lo del rtk, concretamente el del panthom que no te deja o no he adivinado cómo hacerlo con las coordenadas de las imágenes que me proporciona.

He hablado con mucha gente y está  de acuerdo conmigo en que Pix4d debería tener en cuenta ésta opción.

Un saludo

• Marco (Pix4D)

Hola Miguel,

Si su sistema de coordenadas no aparece en la lista, puede agregar el archivo *.prj.

How-to-select-change-the-Image-GCP-Output-Coordinate-System!
Select Image / GCP / Output Coordinate System

Espero que esto ayude,

Saludos

• Miguel Benguría arana

Hola Marco, no me refería al sistema de coordenadas, proyección UTM, mi pregunta era relativa al sistema de coordenadas vertical, es decir, a meter un geoide local. Lo mismo que se puede insertar un sistema de proyección, en mi caso wgs84 28N, que se pueda insertar el geoide, en este caso de las Islas Canarias. La opción que da pix4d son geoides globales y si se busca precisión no sirve en mi caso. Un saludo

• Daniel (Pix4D)

Hola Miguel,

Por el momento Pix4D no soporta modelos locales de geoide. Es por eso que recomendamos convertir las alturas antes de cargarlas en Pix4D si no se quiere perder precisión.

Un saludo.

• hkc

Hi Pix4d. I would like to have the possibility of putting in ellipsoid heights and being able to work with local geoids.

Kindly regards Henrik

• Marco (Pix4D)

Hi Henrik,

Pix4D does not support local geoids at the moment. You need to convert the heights before importing them in Pix4D.

Sorry about that. If there is any improvement on that, we will for sure let you know.

Best

• vuelo 90vuelo

Hola, buenas tardes.

Tengo una diferencia de altura en el MDT generado con un vuelo realizado con un Panthom 4 rtk con respecto al MDT generado con el vuelo realizado con un Panthom 4 pro. Al vuelo del RTK lo procese sin puntos de apoyo y al vuelo hecho con el Panthom 4 pro lo procese con 10 puntos de apoyo.

La diferencia parece ser una traslación en el eje z de mas o menos 1,60 metros.

Porque se estaría produciendo esta diferencia ?

Gracias.

• Marco (Pix4D)

Hola,
No puedo decirles con esta información qué es lo que va mal.
¿Utiliza el mismo sistema de coordenadas verticales?
¿Cuál es la precisión establecida para las coordenadas verticales?
¿Está procesando sus datos RTK con el método de calibración "Accurate Geolocation and Orientation"?

Por favor, compruebe estas recomendaciones y si no cambia nada, si usted es elegible, por favor envíenos un ticket:
Contact Support

Saludos

• Damian Messerli

Hi,

We using GCPs determined in EPSG 2056 (CH1903+, LV95 und LN02) for our Pix4D-projects. the GCP device use de geoid model in Switzerland (CHGeo2004).

in this cas what vertical coordinate system we have to define for the output coordinate system?

do we have to inserte the N-value (geoid hight)?

the coordinate system as below (CH1903+/LV95 - EGM96 Geoid) makes no sens to me.

we Need LN02 (National precise levelling 1902) Z-coordinates for our projects.

best regards

• Holden (Pix4D)

Hi Damian. You can review the article below which explains when to use the height above ellipsoid feature. It seems like this is the best option in this case.