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!

Travelling half way around the world, we joined forces with colleagues from Fugro’s South East Asia offices in Kuala Lumpur, Malaysia for Map Asia 2010. Our presence was strongly felt and we were awarded Best Exhibitor at Map Asia 2010. During the conference’s technical sessions, Fugro EarthData’s Roy Hill delivered two compelling presentations on GeoSAR solutions, one on land-use and land-cover mapping, and the other on base mapping using PurVIEW in support of NSDI initiatives. The presentations highlighted GeoSAR’s advantages in equatorial regions where persistent cloud cover, triple canopy foliage, and rugged terrain, make traditional mapping solutions impossible.

In addition to the tradeshows, GeoSAR also participated in a “ground breaking” ceremony in Anchorage AK to mark the beginning of the 5-year Alaska Statewide Digital Mapping Initiative (SDMI). GeoSAR is currently wrapping up acquisition in Alaska for the first phase of the project awarded earlier this year to Dewberry through the US Geological Survey. The event was attended by over 200 representatives of the federal, state, local and tribal governments, as well as numerous members of the press. Fugro staff also took the opportunity to educate people about IFSAR and the benefits that the resulting maps offer to the state.

After wrapping up a very successful month our conference trail continues with planned stops in Africa, Europe and South America. Please feel free to request further information or continue the conversation at info@geosar.com or leave a comment!

The cover image represents a Digital Elevation Model generated from GeoSAR’s P-band radar overlaid with X and P-band orthorectified images, all in the ChromaDepth® color scheme. The orthorectified images were filtered to increase homogeneity, reduce speckling and remove artifacts. Waterways were flattened and rendered monotonic. The final images were composed into the scene using ESRI®’s ArcGIS software using a custom color pallet that allows the cooler colors to recede and warmer colors to advance on the eye when viewed using the ChromaDepth® 3-D glasses.

GeoSAR’s X and P-band orthorectified images are arranged split screen to highlight features such as terrain, agricultural fields and mangroves. P-band is located in the upper left as the X-band is located in the lower right. P-band highlights features associated with human settlements such as agricultural fields, irrigation channels, roadways and buildings, even those that may be hidden below the vegetation, whereas the mangroves appear brighter in the X-band imagery because they scatter more of the radar energy back.

GeoSAR is the world’s only dual-band, single-pass airborne interferometric SAR system. Penetrating clouds and foliage, GeoSAR simultaneously maps surface features (using x-band) and near bare-earth elevation (using P-band), making it particularly well suited for equatorial mapping.

In GeoSAR mapping we typically use two types of ground control; a LiDAR profiler on the aircraft, and corner reflectors on the ground. As we fly over an area, our LiDAR profiler collects millions of 3-dimentional points (X,Y,Z) with high fidelity. These points provide a highly accurate dataset to compare and validate the GeoSAR products. Corner Reflectors, on the other hand, are deployed in the project area prior to our data collection. Corner Reflectors are targets constructed of material that is highly reflective to the radar signal. These reflectors are surveyed with a high level of accuracy and then imaged by the radar as we fly over them. They provide an easily identifiable known location in the radar data. Reflector data are used in the least-squares adjustment, removing slight offsets in the data between bands, and as another source of validation. The image below shows a radar reflector used during one of our mapping projects. You will notice that there are four sides to the reflector, allowing the reflector to be imaged from multiple sides during a single collection.

Ground control points are just one attribute of our unique GeoSAR mapping solution. If you would like to continue the conversation or receive more information please leave a comment. Stay tuned for more GeoSAR capabilities and updates!

So what is metadata and why the strong reaction? Metadata is data that describes the definition, structure, and administration of other data files. It is, quite literally, data about data. Creating detailed metadata has a reputation as a difficult and painstaking process that requires numerous workers toiling long into the night. However, I’ve worked with a lot of data over the years and I thought I knew different.

To my mind, metadata is an essential part of analysis. Sure, it may be difficult to produce good metadata, but it is so valuable when done right. I decided to dig a little deeper and found a metadata champion to back me up – Fugro EarthData’s mapping specialist, Larry Lund. He’s an expert in data analysis and interpretation and he relies upon accurate metadata everyday. “First of all, you can’t import data into a GIS tool without knowing the projection and coordinate system information and that’s part of standard metadata.” He said, adding, “You may also need to know when this data was captured or its resolution and accuracy – interpreters need that information. It just saves everybody a lot of time”.

I think he’s right. Metadata also documents the origin of data or the production techniques that produced it, essential information for any type of geospatial data analysis. That’s why detailed metadata is at the center of every GeoSAR delivery.

As for the idea that metadata seems hard to produce, it turns out that GeoSAR’s metadata production process is actually a highly-automated program that crawls the network and analyzes the individual files that make up a data delivery. The programming routine performs actions such as “count the number of pixels” or “determine post-spacing”. Once the routine has extracted the information, GeoSAR easily produces FGDC compliant metadata in a variety of formats and delivers it along with binary data.

Now if I could just get some help with spreadsheet macros.

Finally, if you’re planning to attend the INTERGEO- East conference in Instanbul, Turkey, or the GEONT Tech Days in Washington DC this month, be sure to drop by the Fugro booth to learn more about our unique geospatial solutions.

GeoSAR Crossword Puzzle Answers

GeoSAR Crossword Puzzle

The overall concept isn’t exactly new. Ground penetrating radar (GPR) technology, which utilizes radar bands in the UHF/VHF frequencies from hand-held or vehicular-based equipment, is a well established technique. It’s the use of remote sensing platforms for subsurface mapping that is gaining ground (no pun intended!). Using satellite and airborne methods, it is possible to provide wide-area coverage of subsurface hydrology and structures for use in mineral exploration and development, infrastructure design, and archeological research.

So where is the science? In the early 1980s, the hyper-arid Bir Safsaf region in southwestern Egypt was mapped using Shutte Radar SIR-A and SIR-B to detect geologic structures covered by layers of sand. Discoveries from that research included fossil river systems. More recent studies have shown the ability of very low-frequency radar, such as P-band, to penetrate depths of 15 meters or more depending on soil composition, radar incidence angles, and soil moisture.

Fugro EarthData has confirmed these P-band findings using GeoSAR during a mission over the Mojave Desert near Edwards Air Force Base. Among other things, the results revealed a mostly submerged geological formation. Subsurface mapping is an area we’ll continue to research given its many implications related to water networks, national security, and heritage management. It’s also one more way that we can help clients maximize the value of their GeoSAR data—by unlocking what we call, “the Power of P”.

Want to continue the conversation? Leave a comment or send us an email at info@geosar.com.