You probably have GPS in your smartphone (and possibly your camera or other devices) but I'm going to focus on hand-held 'recreational' GPS units. This type of GPS receiver can be purchased for under $100 or you might spend as much as $400 (more than that and you are looking at a different class of device). I'm currently working with a Garmin GPSMap 64s that I bought for $230. Over the last 15 years I've purchased three similar devices:
- The GPSMap 64s - purchased in Jan. 2016, referred to throughout as the '64s'
- A Garmin eTrex Venture HC. Six or seven years old, the 'eTrex'. Around $130
- A Magellan eXplorist 210. Fifteen years old. I no longer use this device.
GPS receivers like these are typically described as being accurate to 10 meters (30 feet). But that is the kind of safe generalization that manufactures make to avoid legal disputes. Under favorable conditions these units common fix locations within 10 to 15 of the actual location. Under ideal conditions these units can provide coordinates that are within a few feet of the exact location. If an even higher level of accuracy is needed you can purchase GPS receivers that come with very sensitive external antennas and that make use of ground-based correction signals and post processing of the raw GPS data. At the highest end these unit achieve sub-centimeter accuracy (better than half an inch). These units currently cost thousands of dollars.
But how do you judge the accuracy of locations saved using your GPS receiver? One way is to upload the 'marks' or 'waypoints' captured by your GPS device to a computer for display on top of a map or geographically referenced imagery. I do this using Google Earth. To judge accuracy at the level of a few feet you'll need to record locations at a place that you can identify precisely on a map. The place that I'm using for these tests is seen in Image One. This satellite view from Google Earth shows a complex of buildings located in the Saratoga Spa State Park near my home. The square in the center of the image is a wading pool (it was empty when this satellite image was made). Image Two shows the wading pool with the map zoomed in for a closer look. In Image Two I added a marker place precisely on one corner of the wading pool. The properties dialog then shows the geographic coordinates for that location.
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| Image One: Building complex in Saratoga Spa State Park. The square shape in the center of the image is a wading pool. For scale, the wading pool is 93 feet on a side. Image captured from Google Earth. |
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| Image Two: The wading pool again, this time zoomed in for a closer view. A marker was added and the properties dialog shows the geographic coordinates of the marker (in the Decimal Degrees format). Coordinates captured using GPS can be compared with a known location such as the corner of the pool to judge accuracy. Image captured from Google Earth. |
To get really accurate locations your device needs to make use of signals from ground-based stations. These stations broadcast signals that your GPS receiver can incorporate into its' calculations and these stations don't move. The location of the ground based stations is known with great precision. Some consumer GPS receivers, including the Garmin GPSMap 64s, can use the WAAS ground correction network but I'm not clear on when this is used and the effect that it has on the accuracy of locations recorded using the 64s, The underlying problem with ground-based correction is that you need to be within the range and coverage area of a transmitter. Currently this is common in urban areas and uncommon away from cities. [Note: I've really over-simplified this, follow the link above for more detailed information]
Really good accuracy also requires the use of a highly sensitive antenna and this is probably the most significant factor in limiting best case accuracy in low cost GPS receivers. This is especially true when using GPS in the dense forests of the Adirondacks. Your GPS receiver must receive signals from at least four satellites (three provides a rudimentary fix) and these signals can be blocked by a dense tree canopy as well as by mountains or buildings. Intriguingly, the GPSMap 64s has a port that allows for the use of an external antenna. I have not yet begun to explore what that might mean in terms of the best accuracy level this receiver can achieve.
Really good accuracy also requires the use of a highly sensitive antenna and this is probably the most significant factor in limiting best case accuracy in low cost GPS receivers. This is especially true when using GPS in the dense forests of the Adirondacks. Your GPS receiver must receive signals from at least four satellites (three provides a rudimentary fix) and these signals can be blocked by a dense tree canopy as well as by mountains or buildings. Intriguingly, the GPSMap 64s has a port that allows for the use of an external antenna. I have not yet begun to explore what that might mean in terms of the best accuracy level this receiver can achieve.
But, without further ado, let's look at some comparisons. I recently did an informal comparison of the accuracy of locations recorded using a Garmin GPSMap 64s compared results obtained using the older Garmin eTrex. I marked the same locations using the GPS in my smartphone to provide an additional set of comparisons. Image 3 shows the results.
I recorded the location of each of the four corners of the wading pool shown in the images above using each device. Since the corners have indents I marked the exact locations with the red circles to show where I took the measurements. Locations recorded using the 64s are represented by the green markers (note that the marker points are the at the recorded location). The locations recorded using the eTrex are represented by the purple markers. And the locations recorded using my phone are represented by the pink markers. As you can see, the 64s and eTrex achieved similar levels of accuracy. The eTrex was actually the winner; the mark closest to the physical location was recorded by the eTrex and that mark is about five feet away from the map image location. However, the 64s was more consistent and that also could be used to decide which device is "more accurate." This was a bad day for the phone. I usually get decent results using the GPS in my phone but on this day at that place it was all over the map. One mark, labeled Phone-3, seen on the left side in the image, was off by almost 200 feet. It should have been around the corner of the wading pool at lower right.
The extra mark at the upper right, labeled "E4b" was recorded by the eTrex and I included it to support my contention that the 64s was more consistent. I actually recorded two marks on each device; standing in the same spot. These marks were generally very close to each other but in this one case the first mark recorded by the eTrex was 25 feet away. Inconsistent.
A few conclusions can be drawn from this test. For starters, while the place and conditions at the time I recorded the data were favorable, the results are not particularly impressive. Both of the Garmin receivers marked the locations at points within ten feet of the actual, but only a couple were with five feet. I plan to return at a different time of day to repeat the test to see how much the results vary. Different levels of accuracy are to be expected because at different times your receiver will obtain GPS signals from different satellites. That the eTrex unit produced the closest marks could actually be an accident. I've been comparing the fixes obtained using both devices informally for over a month and in nearly every case the 64s has performed better. As the title of this post says, GPS accuracy is not a simple matter.
Recording fixed locations under controlled conditions is interesting, but my main use for these devices is to record tracks while hiking in the Adirondacks. Image four shows a portion of a track recorded by both devices on a recent outing. The light blue line represents the track recorded by the 64s and the purple line is from the eTrex. The 64s track is more accurate and more precise. This comparison is interesting because it removes bias from the comparison (More on that in a future post).
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| Image Three: |
I recorded the location of each of the four corners of the wading pool shown in the images above using each device. Since the corners have indents I marked the exact locations with the red circles to show where I took the measurements. Locations recorded using the 64s are represented by the green markers (note that the marker points are the at the recorded location). The locations recorded using the eTrex are represented by the purple markers. And the locations recorded using my phone are represented by the pink markers. As you can see, the 64s and eTrex achieved similar levels of accuracy. The eTrex was actually the winner; the mark closest to the physical location was recorded by the eTrex and that mark is about five feet away from the map image location. However, the 64s was more consistent and that also could be used to decide which device is "more accurate." This was a bad day for the phone. I usually get decent results using the GPS in my phone but on this day at that place it was all over the map. One mark, labeled Phone-3, seen on the left side in the image, was off by almost 200 feet. It should have been around the corner of the wading pool at lower right.
The extra mark at the upper right, labeled "E4b" was recorded by the eTrex and I included it to support my contention that the 64s was more consistent. I actually recorded two marks on each device; standing in the same spot. These marks were generally very close to each other but in this one case the first mark recorded by the eTrex was 25 feet away. Inconsistent.
A few conclusions can be drawn from this test. For starters, while the place and conditions at the time I recorded the data were favorable, the results are not particularly impressive. Both of the Garmin receivers marked the locations at points within ten feet of the actual, but only a couple were with five feet. I plan to return at a different time of day to repeat the test to see how much the results vary. Different levels of accuracy are to be expected because at different times your receiver will obtain GPS signals from different satellites. That the eTrex unit produced the closest marks could actually be an accident. I've been comparing the fixes obtained using both devices informally for over a month and in nearly every case the 64s has performed better. As the title of this post says, GPS accuracy is not a simple matter.
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| Image Four |
Recording fixed locations under controlled conditions is interesting, but my main use for these devices is to record tracks while hiking in the Adirondacks. Image four shows a portion of a track recorded by both devices on a recent outing. The light blue line represents the track recorded by the 64s and the purple line is from the eTrex. The 64s track is more accurate and more precise. This comparison is interesting because it removes bias from the comparison (More on that in a future post).
My contention that the the tracks recorded by the 64s are more accurate comes from knowing the details of this outing. In the lower part of the image, the four distinct lines should overlap almost perfectly because that portion of the route follows a path that I walked along on the inbound portion of the hike and then again, in the opposite direction, on my return. My actual route might have varied by a few feet in places but those lines should basically overlap. In places there could be differences of a few feet between the inbound and outbound tracks but the two tracks should nearly overlap.
Measuring the gap in Google Earth I find that the it averages about 15 feet for the track recorded by the 64s and by 40 feet for the track recorded by the eTrex. In this context the 64s was significantly more accurate.
Measuring the gap in Google Earth I find that the it averages about 15 feet for the track recorded by the 64s and by 40 feet for the track recorded by the eTrex. In this context the 64s was significantly more accurate.
