The earth is shaped like this, with the north and south poles acting as the top and bottom points of an approximate egg. Recall that an “ellipsoid” is a 3D shape like a sphere, but more like an oval. Michael’s GPS receiver is referencing a theoretical representation of the earth’s surface called the “ellipsoid.” But because the earth’s surface is full of physical anomalies and is changing constantly, scientists rely on theoretical representations - called “vertical datums” - to represent the earth’s surface.
But to understand why his elevation data seems wrong, we need to understand what kind of “elevation” his GPS receiver is measuring.Īll elevation measurements are, in essence, the difference between the earth’s surface and a point above or below it. Is the GPS receiver wrong? Part 1: The “Ellipsoidal Model” - Your GPS receiver measures elevation with respect to “the ellipsoid” The ellipsoid reference His GPS receiver elevation data was off by tens of meters.
Michael recorded his elevation measurement from his devices, compared it with the published coordinates, and was surprised. In the United States, you can find information on such monuments in your area via the National Geodetic Survey (NGS) website. A survey monument is a point with published high accuracy horizontal and vertical coordinates. So, Michael took his iPad ® and Arrow Gold into the field and tested them by recording the elevation of a survey monument.
He bought a new iPad mini ®, installed Esri’s ArcGIS Collector, and configured a high-accuracy Arrow Gold GNSS receiver via Bluetooth to create the perfect, easy-to-use field data collection solution.īut before sending his field crew out with the technology, Michael wanted to make sure the solution worked. To produce accurate (within centimeters) elevation data required, Michael is tasked with surveying the utility’s water assets in the field. As such, Michael is responsible for ensuring its field technicians collect accurate asset location data, including elevations. Michael is the (fictional) GIS analyst at an (also fictional) U.S. What is the difference between the ellipsoid, mean sea level, geoid, geoid height, and orthometric height - and which one is right?
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