GPS Accuracy: Part Two

This is a follow up to my previous post on GPS accuracy. Basically, I returned to the same location --the wading pool in the Roosevelt Baths Quad at Saratoga Spa State Park-- and recorded additional sets of GPS fixes using the Garmin GPSMap 64s and the Garmin eTrex. Image One is a screen capture from Google Earth showing the results. The green markers represent the fixes recorded using the 64s and the red markers were recorded using the eTrex. The red dots represent the actual locations.

Image One: Screen capture from Google Earth. The green markers represent locations recorded by the Garmin GPSMap 64s and the red markers are from the Garmin eTrex Venture HC. The red dots show the points where I stood to record the locations.

My intent, as described in detail in the previous post, is to understand the level of accuracy that can be obtained using inexpensive (less than $300) GPS devices and to determine the factors that affect that accuracy. From that perspective, the average accuracy of the fixes recorded on this visit to the Spa State Park site are similar to those obtained on the previous visit. Slightly better overall, but not substantially different.

The majority of the fixes recorded by both units where within ten feet of the actual and both recorded some fixes within five feet of the actual. Using the criteria discussed in the previous post these are excellent results. My methodology is to record a fix with each unit, walk to the next location and repeat the process. Starting at the upper left I proceeded to the lower left, lower right, and upper right corners of the pool. The green marker labeled 001 was the first fix recorded using the 64s and the red marker labeled 001 was obtained a few seconds later using the eTrex. I made three circuits so there are three markers for each unit at each corner.

On the third circuit I used the location averaging feature available on both devices. When using averaging the GPS device grabs a sequence of fixes at the location and averages those fixes to produce a single fix that should be more accurate. I have not seen a description of the algorithm these devices use to do the averaging, but an educated guess is that the units temporarily store fixes obtained using different sub-sets of the full set of satellites from which the unit is receiving signals. Comparing these trial fixes would allow the unit to drop signals that don't fit well with the others. See the previous post for a discussion of conditions that can reduce GPS accuracy.

To make it easier to see how these results compare I grabbed an additional screen capture showing a closer view of the lower left corner of the pool (the second stop on the circuit). And this brings me right back to the title of the first post; GPS accuracy is not a simple matter. As you can see, the distance from the Green marker 010 to the actual is 1.3 feet as measured using the Google Earth measurement tool. That fix, obtained on the 3rd circuit of the pool using the averaging feature of the 64s, was the most accurate fix obtained by either unit. But the 64s also recorded one of the worst fixes of the day at that same location. The green 002 marker, recorded by the 64s on the first circuit of the pool, is 16 feet away from the actual. That's not terrible but it is the 2nd worst result for either device on any of the three circuits. Given that the second time around the 64s recorded a fix about five feet away from the actual (green 006) it was probably the case that the 002 marker incorporated a signal from a satellite with corrections pending, and that that through off the result.

In summary, the accuracy you obtain using inexpensive GPS devices will almost always be better than the 30 foot level that the manufacturers claim for these devices. And it can be much much better with best case results under five feet. But that level of accuracy requires an understanding of how these devices work and what factors reduce accuracy. To obtain these 'best case' results you should turn your device on several minutes before recording the first fix -this allows your GPS device to download the satellite correction data it needs to make proper use of each satellite. Secondly, use the averaging feature of your GPS if it has one. Or, take several readings at the same location a few seconds apart and do the averaging yourself.

Most important, be skeptical. It may be that the one foot accuracy I got on this go round is a fluke. After all, I didn't see that on my first visit to the same site. Leaving me to wonder, what will the next visit show?

Image Two: Screen capture from Google Earth. The 1.3 feet distance from the actual location to the green marker 010 (third circuit) was most accurate fix by either unit. Confouding that, the green 002 marker, also recorded by the 64s, was one of the worst.