How do you use geodetic datums in GPS surveying?

1 What are geodetic datums?

Geodetic datums are mathematical models that describe the Earth's surface and gravity field. They are based on physical observations, such as the position of stars, satellites, and monuments, and theoretical assumptions, such as the shape of the Earth and its rotation. Geodetic datums consist of two components: a geoid and a datum surface. A geoid is an imaginary surface that represents the mean sea level and the direction of gravity. A datum surface is a simplified approximation of the geoid, usually an ellipsoid or a sphere, that fits the geoid as closely as possible.

2 Why are geodetic datums important for GPS surveying?

GPS surveying relies on the use of satellites that orbit the Earth and transmit signals to receivers on the ground. These signals contain information about the location and time of the satellites, which can be used to calculate the position of the receivers. However, GPS coordinates are not directly compatible with the coordinate systems that civil engineers use for mapping and design, such as UTM, State Plane, or local systems. To convert GPS coordinates to these systems, you need to use geodetic datums that define the relationship between the satellite and the terrestrial coordinate systems.

3 How do you choose a geodetic datum for GPS surveying?

When selecting a geodetic datum for GPS surveying, several factors must be taken into consideration, such as the location, scale, and purpose of the project, as well as the availability and compatibility of data sources. WGS 84 is a global datum used by the GPS system and is based on an ellipsoid that approximates the shape of the entire Earth. This datum is regularly updated to account for changes in the Earth's crust and rotation. NAD 83 is a regional datum used for North America and is based on an ellipsoid that fits the North American continent better than WGS 84. It is aligned with the North American Plate. ETRS 89 is another regional datum used for Europe which is based on an ellipsoid that fits the European continent better than WGS 84 and is aligned with the Eurasian Plate.

4 How do you apply a geodetic datum to GPS surveying?

Applying a geodetic datum to GPS surveying involves two steps: transformation and projection. Transformation is the process of converting GPS coordinates from one datum to another, using mathematical formulas or tables that account for the differences in the shape, size, and orientation of the datums. Projection is the process of converting GPS coordinates from a three-dimensional ellipsoidal system to a two-dimensional planar system, using geometric methods that preserve certain properties, such as distance, area, or direction.

5 What are the challenges and limitations of geodetic datums in GPS surveying?

Geodetic datums in GPS surveying are not perfect or fixed, and can be subject to errors, uncertainties, and changes over time. One of the challenges is datum mismatch, which occurs when different data sources use different datums or the same datum is updated or revised. This can lead to discrepancies and inconsistencies in GPS coordinates and measurements. Datum distortion is another issue, which happens when the datum surface does not match the actual shape of the Earth or local topography. This can cause errors and distortions in GPS coordinates and measurements over large areas or high elevations. Lastly, datum movement occurs when the surface shifts or rotates due to tectonic, geophysical, or gravitational forces. As a result, this can lead to changes and variations in GPS coordinates and measurements over time.