Consumer-Grade GPS Receivers for GIS Data Collection
I hereby proclaim June GPS/GIS month (at least for me). I’m dedicating the next three newsletter columns (early June, mid-June, and early July) and a webinar (June 30) to discussing using GPS receivers and technology for GIS (geographic information systems) data collection. Why, you may ask?
First of all, I realize my domain is typically the high-precision survey/construction arena, but the boundary isn’t so clear cut any longer. Many surveyors, engineers and construction crews use less accurate GPS receivers for activities such as GIS data collection, recon, and navigating — so the topic is relevant.
Secondly, ’tis the season. The ESRI User Conference is in mid-July this year — about six weeks from now. Although high-precision GPS has a firm place there and is growing, the ESRI UC is the largest conference in the world where non-survey GPS is near center stage. It is one of the primary data-gathering tools that fuels a GIS.
There have been some really significant changes in the last 10 years. GPS data-collection tools for GIS have expanded. At that time, consumer receivers couldn’t be used because Selective Availability (SA), the intentional degradation of GPS accuracy by the Department of Defense, was still active. Also, “submeter” GPS mapping systems were backpack-based, contained a “rat’s nest” of cables, required camcorder batteries to run, and were generally bulky. Data collectors were based on DOS instead of Windows. Lastly, users were primarily using post-processing to differentially correct their GPS data or using Marine DGPS/NDGPS in select locations or commercial DGPS services like OmniSTAR for real-time DGPS.
Fast forward to today. Three categories of GPS are being used to populate GIS databases: consumer-grade receivers, GPS receivers designed specifically for GIS data collection, and survey receivers used for GIS data collection. In this column, I’ll discuss using consumer-grade receivers for GIS data collection. In my mid-June column, I’ll discuss the class of GPS receivers designed specifically for GIS data collection.
Overnight, when SA was turned off in May 2000, consumer-grade GPS receivers became a viable option for GIS data collection where accuracy is not of the highest priority. Today, due to improvements to the GPS itself as well as GPS receiver technology and along with the maturation of WAAS/SBAS, consumer-grade GPS accuracy is even better.
Thousands, maybe tens of thousands, of consumer-grade GPS receivers are being used to collect data used for GIS. They are easy to use and the price is attractive.
Understanding the accuracy of a consumer-grade GPS receiver is not a simple task. In fact, if you’re not careful, you can be easily misled. For example, take a receiver out to the parking lot and wait for it to obtain enough satellites and a WAAS/SBAS correction. You may be impressed with its precision as it might be within a couple of meters or even better. There are two issues with this:
- Repeatability…accuracy vs. precision. Precision is a group of points that are tightly clustered but not in the right place. For example, you may have a cluster of 10 points all within two meters of each other, but they are five meters from the true location. This is not necessarily desirable, but quite typical for consumer-grade GPS receivers. Some receivers offer an “EPE” (Estimated Positional Error) value on the display to provide you and indication of accuracy. Absolutely do not rely on this value in an attempt to estimate the position accuracy of the receiver. It is a rough guess at best.
- Performance in less-than-desirable GPS conditions. Surprisingly, or not, users assume that performance in a grove of trees is going to be similar to performance in a parking lot with a wide open view of the sky. This is not the case.
I’ll give you a real case study. Several years ago I was helping a company setup a GPS system to map utility poles. Their required accuracy was +/- 3 meters. A local survey equipment salesperson suggested they use a consumer-grade Compact Flash (CF) GPS receiver plugged into the top of a ruggedized PDA. The salesperson demonstrated the receiver in the client’s parking lot. The performance, in the client’s eyes, seemed like it would meet the +/- 3 meter requirement. The price was right at $250 per receiver and they need upwards of 15 receivers. There were a couple of alternative proposals that cost significantly higher per receiver ($2,000-$4,500 each). The price difference was too great for the client not to be tempted to try the $250 receiver so they purchased six of them. They ended up using them for only 60 days. The bottom line was that the receiver performed very poorly in the field in two areas. First, many of the utility poles were located in areas where there were many trees. The client found that the CF GPS receiver performed very poorly in that environment. Some positions were off by more than 50 meters. Secondly, the client found that the CF GPS receiver had a difficult time maintaining lock on the WAAS satellites used for corrections even in relatively wide open areas where this shouldn’t have been a problem.
In this case, the lesson is to try the receiver in an environment where you will be using it. All GPS receivers will perform worse under tree canopy as compared to their performance in an open area. This is the Achilles heel of GPS. That being said, some GPS receivers perform better under tree canopy than others. The ones that do perform better under trees were designed to do so. Using a consumer-grade GPS in that environment is sort of like trying to compete in a Formula One race with a Volkswagen Beetle. The design criteria of the Beetle was fuel economy and low cost, not acceleration and cornering. The same applies to consumer GPS receivers. Accuracy is not one of the top criteria for consumer GPS receiver designers. They are much more concerned with low cost, low power consumption, small antenna size and fast satellite acquisition, as they should be. My wife, for example, really doesn’t care if it’s accurate to 15 meters vs. 1 meter as long as she arrives at the destination she plugs into the system. On the other hand, high-performance GPS receivers designed for GIS data collection sacrifice some features such as power consumption, antenna size, and small size in order to optimize accuracy.
This is not to say that consumer GPS receivers have no place in GIS mapping. On the contrary, they have a very important place. My point is that your expectations should match reality when evaluating receivers to use for your project. The accuracy specifications on consumer GPS receiver datasheets are essentially meaningless. The only way to truly understand the performance of a particular receiver is to try it yourself.
One final note on this. Many commercial (typically survey equipment dealers) and academic entities have published accuracy comparisons of different consumer GPS receivers. You really have to take these reports with a grain of salt. Sometimes the reports are intentionally biased and other times they are biased due to lack of knowledge or experience. They are also based on an environment that may not be similar to yours. “Heavy” tree canopy is a subjective term. Tree canopy in Oregon is different than tree canopy in Alberta and is different from tree canopy in Austria.
The Final Analysis
- Low cost
- low power
- Poor accuracy in challenging GPS conditions
- inconsistent accuracy in non-challenging GPS conditions
- unable to post-process (with a few exceptions)
- no on-board GIS data collection functionality