Fundamentals of Surveying (FS) Practice Exam

What is the limiting factor when obtaining elevations with GNSS?

1. A clear line of sight

2. A precise ellipsoid model

3. A precise geoid model

4. A precise gravimetric model

The correct answer is: A precise geoid model

When obtaining elevations using GNSS (Global Navigation Satellite System), the limiting factor is a precise geoid model. This is because GNSS measures heights relative to an ellipsoid, which is a mathematical surface that approximates the shape of the Earth. However, for practical applications like surveying, what is often required is the height above the geoid, which represents mean sea level. To convert GNSS-derived ellipsoidal heights into orthometric heights (the height above the geoid), a precise geoid model is necessary. The geoid model provides the relationship between the ellipsoid and the geoid, allowing surveyors to make accurate height measurements. Without an accurate geoid model, the conversion from ellipsoidal heights to orthometric heights can lead to significant errors in elevation data. In contrast, while a clear line of sight might be important for satellite connectivity, it is not the primary limiting factor when it comes to elevation accuracy. Similarly, precise ellipsoid and gravimetric models contribute to understanding the Earth's shape and gravity field, but they do not directly address the critical need for an accurate geoid model in the context of surveying elevations.