What issues arise in relation to Altitude Measurement, NAP, and the Geoid?
- What exactly do we mean by “height” in geodesy?
- How is elevation traditionally determined using levels, and how accurate is this method?
- What is the Amsterdam Ordnance Datum (NAP), and how is this height reference system maintained?
- Why does GNSS provide a different type of elevation than the one we use in the Netherlands relative to NAP?
- What is the geoid, and why is it necessary to make GNSS heights usable?
- What errors and uncertainties are involved in determining and using elevations?
During this Blended Learning course, you’ll learn about the principles behind elevation measurement and elevation references. You’ll discover that elevation is less straightforward than it seems and that different methods yield different types of elevation.
The focus is on the relationship between traditional height measurement, GNSS, the NAP, and the geoid. You’ll learn how these elements are interconnected and why a solid understanding of height references is essential for surveying and other Geo-ICT applications.
What do we mean by elevation?
When we say that a point has a certain elevation, it must always be clear relative to what that elevation is determined. Within geodesy, there are therefore different types of elevations and different reference planes.
You’ll be introduced to concepts such as ellipsoidal height and physical height. You’ll learn why a height determined directly using GNSS is not the same as a height relative to mean sea level or NAP.
This makes it clear why, when working with elevation data, it is important to consider not only the elevation value but also the elevation reference system used.
Elevation Measurement with Levels
Leveling is one of the classic and most accurate methods for determining elevation differences between points. In this method, the elevation difference between various points is measured along a horizontal line of sight.
You’ll learn the basics of how a leveling survey is conducted and how elevation differences over longer distances can be determined through a series of measurements. You’ll also be introduced to concepts such as the forward benchmark, rear benchmark, elevation difference, and closure error.
In addition, you’ll examine potential errors and uncertainties during leveling and the checks performed to assess the quality of the measurements.
The goal of this Blended Learning course is not to conduct extensive field measurements with leveling instruments. Rather, it is to help you understand how elevations are determined and how the quality of elevation measurements can be verified.
NAP as the Dutch Height Reference
In the Netherlands, elevations are generally expressed relative to the Amsterdam Ordinary Sea Level (NAP). This serves as the national reference for elevation measurements and plays an important role in areas such as water management, infrastructure, construction, and surveying.
You’ll learn how the NAP came to be, how the Dutch elevation network is structured, and how elevations are transferred from reference points to other locations.
You’ll also discover why a reliable elevation reference system is particularly important for the Netherlands. In a country where large areas lie at or below sea level, accurate and consistent elevation data are essential for water safety and infrastructure management, for example.
The Geoid and the Gravitational Field
The Earth does not have a simple geometric shape. Due to differences in mass distribution, the gravitational field varies, creating a physical reference surface that we call the geoid.
You will be introduced to the concept of the geoid and learn why it plays an important role in determining physically meaningful elevations.
The connection to the ellipsoid used in GNSS will also be explained. The ellipsoid is a mathematical model of the Earth, while the geoid is related to the Earth’s gravitational field.
You do not need to mathematically model the gravitational field yourself. The goal is for you to understand why the geoid is necessary and why the distance between the geoid and the ellipsoid varies by location.
From GNSS Height to NAP Height
GNSS makes it possible to quickly determine a three-dimensional position. However, the height that a GNSS receiver directly determines is an ellipsoidal height.
For many applications in the Netherlands, however, an elevation relative to NAP is required. To convert GNSS elevations to usable NAP elevations, a geoid model or elevation transformation model is therefore used.
You will learn how ellipsoidal height, geoid height, and physical height are related. You will also explore what can happen when elevation data from different reference systems are combined without the proper transformation.
This establishes a direct link between GNSS & satellite positioning and the Dutch height reference system.
Elevation Measurement, NAP, and the Geoid in Practice
During the Blended Learning sessions, you’ll work with examples and existing elevation data to link theory to practical Geo-ICT challenges. You’ll compare different types of elevations and examine how elevation data is stored and used within GIS.
You’ll work on assignments such as:
- Investigate the difference between ellipsoidal height and height relative to NAP.
- Analyze a simple leveling survey and verify the measured elevation differences and closure error.
- Investigate how the Dutch NAP network is structured and how elevation reference points are used.
- Compare GNSS elevations with NAP elevations for a number of locations and explain the differences.
- Investigate the role of the geoid in converting GNSS elevations to physically usable elevations.
- Work with elevation data in QGIS and investigate the consequences that an incorrect elevation reference can have on data interpretation.
By the end of this module, you will understand how elevations are determined and recorded in geodesy, the role of the NAP as the Dutch elevation reference, and why GNSS provides a different type of elevation. You will understand the function of the geoid and be able to explain how GNSS heights are linked to usable heights for surveying, water management, infrastructure, and other Geo-ICT applications.
