As a key player in the map-making business, Fugro is always engaged in discussions about precision, accuracy, and reliability standards within the geospatial marketplace. Fugro’s GeoSAR system has unique features that ensure quality standards are always met. These features include 4-look acquisition data redundancy as a primary feature of the GeoSAR dual-sided radar configuration, its large side overlap on adjacent flight lines, and its profiling LiDAR. As discussed in our previous “Ground Control” blog post on ground control on the fly.
Let’s first discuss what these quality terms mean in relation to GeoSAR. Accuracy is an absolute term, describing how close the estimated elevation (or position) at a given point is to the true elevation (or position) of that point. Precision is a relative term, describing the quality of the height difference between two points. Reliability is the ability to detect and correct measurement errors, which depends highly on the redundancy of the measurements.
For example, imagine using an old tape measure with 1/8” markings to measure the height of a table at all four corners. Suppose that the measurements are 30-2/8”, 30-1/8”, 29-7/8”, and 30-1/8” – or equivalent to an average of 30-1/8” with a precision of 1/8”. Suddenly, you discover you are off by an inch. That is accuracy. The measurements were inaccurate by about 1”. This example shows that measurements can be very precise, but not necessarily accurate, or they can be accurate, but not precise.
To prevent such errors, the GeoSAR system uses a calibration campaign to resolve systematic errors using precisely surveyed corner reflectors at known locations on a calibration site. A corner reflector is to radar what a benchmark is to photogrammetry—it provides a very precise geospatial correspondence between a radar point and a {X, Y, Z} location on the ground. This is done to ensure the GeoSAR measurements are accurate.
The precision of GeoSAR, or airborne IFSAR, depends on factors such as the aircraft altitude, amount of turbulence, the separation between flight lines, terrain slope, moisture, and other factors. In typical situations, airborne IFSAR is able to measure terrain elevation and geoposition at meter level precision. The accuracy of GeoSAR products depend on a variety of key factors, including the GPS position of the aircraft, the quality of ground control, and the accuracy of the geoid. Similar to GPS, Fugro GeoSAR elevations are measured in ellipsoidal heights and converted to orthometric heights using a geoid model. The more accurate the geoid model, the more accurate the GeoSAR orthometric height will be.
Understanding and applying these key factors is what separates Fugro’s GeoSAR services and products from other service providers. Stay tuned next week when we discuss Resolution and Posting. If you would like to continue this discussion or would like more information, please leave a comment!
