Tech Chat | What is geoid model and how to use it?
In previous blogs, we explored the distinctions between three common height measurements—geodetic height, normal height, and orthometric height. In our daily RTK surveys that based on reference ellipsoids, we primarily encounter geodetic height. However, sometimes we may need normal height that aligns better with the local terrain.
In this blog, let's learn what the geoid model is and demonstrate how to utilize it effectively in your daily surveying tasks.
If you missed the previous blog, catch up here:
[Tech Chat | What's the difference between geodetic height, normal height, and ortho-height?]
What is a Geoid Model?
A geoid model is a leveling model that converts geodetic height to normal height by considering elevation anomalies. This model is crafted by the National Geospatial-Intelligence Agency (NGA) in collaboration with various agencies, including the National Imaging and Mapping Service (NIMA). The construction of the model relies on surface gravity data from different global regions and aerial gravity surveys in specific areas. Prominent geoid models include EGM84, EGM96, EGM2008, and EGM2020.
The Importance of Geoid Models
In the realm of engineering and construction, our primary focus is on normal height, while GPS measurements provide geodetic height. To bridge this gap, we need to convert geodetic or orthometric height to normal height.
Scheme 1: Orthometric Height → Normal Height
Calculation of orthometric height is influenced by Earth's gravity and poses challenges in obtaining precise values. Additionally, orthometric height is based on the geoid, an idealized model challenging to accurately represent in the real world.
Scheme 2: Geodetic Height → Normal Height
This entails calculating the difference between Earth's height and normal height, often referred to as elevation anomaly.
- Method 1: The traditional approach involves jointly measuring several evenly distributed GPS known points to cover the control area effectively. These known points' coordinates are utilized in post-processing software to constrain the adjustment of observation data, yielding accurate plane coordinates but with elevation accuracy susceptible to terrain and known point distribution. However, this method is time-consuming, expensive, and operationally complex.
- Method 2: Employing a geoid model to determine the normal height. Simply apply the model file to your project, enabling a swift calculation of normal height for your measured points.
Applying the Geoid Model
1. Download the Data: Obtain the relevant data from designated websites and copy the file to your PDA device (e.g., SC100 data collector).
2. Import the Model File: Open SingularPad field software,
- Import the geoid model as below when setting up your surveying project
- If you have already started a project, go to Project >> Coordinate System, then import the geoid model file
Tips: Due to the large size of the global geoid model, you can use professional software to extract and isolate the required country or area from the original file. You can also contact SingularXYZ support team for help support@singularxyz.com
Case Comparison
In conclusion, understanding and effectively utilizing geoid models in surveying can significantly enhance the accuracy and efficiency of height measurements, ensuring that projects align with the local terrain and meet specific project requirements.
Learn more about SingularXYZ SingularPad field software:
https://www.singularxyz.com/SingularPad.html
