But not to worry, that's not what this post is all about. I have four devices that incorporate GPS in some way and I'd like to compare the relative accuracy of the four. I'd also like to know which one is most accurate and what conditions alter that accuracy level.
The four devices that I'm evaluating:
The four devices that I'm evaluating:
- A Garmin eTrex Venture HC hand-held GPS - (from 2009, now discontinued, but similar models are available for around $125, see the Garmin eTrex 10, 20, 30)
- A Magellan Explorist 210 (from roughly 2002, long ago discontinued)
- A Samsung Charge smart phone (2.5 year old Android smart phone)
- A Canon Powershot sx260HS camera with built-in GPS
The testing was done under ideal GPS operating conditions;
- Clear skies
- In the middle of a large open area with no obstructions
- Plenty of time for each device to lock signals from as many GPS satellites as possible
The Basic Test
My testing will consist of recording latitude and longitude pairs from each device at known locations so that the coordinates returned by the devices can be compared with the actual coordinates. So how am I finding these "known locations"? I'm using Google Earth to zoom in on an easily recognized location near my home. I then add placemarkers in GE and view the coordinates that GE assigns to those placemarkers. So, yes, there are assumptions being made. For one, I'm assuming that the coordinates I obtain from Google Earth are trust worthy and two I'm discounting (for now) any discrepancies that might come from the use of different coordinate systems by the devices involved.
My testing will consist of recording latitude and longitude pairs from each device at known locations so that the coordinates returned by the devices can be compared with the actual coordinates. So how am I finding these "known locations"? I'm using Google Earth to zoom in on an easily recognized location near my home. I then add placemarkers in GE and view the coordinates that GE assigns to those placemarkers. So, yes, there are assumptions being made. For one, I'm assuming that the coordinates I obtain from Google Earth are trust worthy and two I'm discounting (for now) any discrepancies that might come from the use of different coordinate systems by the devices involved.
The location for the testing is a square wading pool (82 feet on a side) located in a “quad” complex in the Saratoga Spa State Park inSaratoga Springs, NY. Figure One shows a screen capture of the pool as it appears in Google Earth. Figure two shows the pool with more of the surrounding area visible to provide context. I made three circuits of the pool stopping at each corner and recording the location with each of the four devices. I then averaged the latitude and longitude values for each each device to obtain a single location per device. These coordinates were used to add points markers to Google Earth and I could then use the Google Earth measuring tool find the distance between the locations provided by each device and the reference location. Those measurements are shown in Table One.
 Wading Pool in Saratoga Spa State Park as seen in Google Earth. GPS fixes were obtained at each of the four corners. The image on the right shows the wading pool in context. |  * |
Figure One
Garmin
|
Magellan
|
Phone
|
Camera
| |
Difference between GPS location and GE coordinates in Feet
|
8
|
6
|
25
|
10
|
Difference between the GPS elevation and elevation obtained from Google Earth
|
1
|
10
|
94
|
20
|
No. of satellites registered by each device
(after minimum 5 minute warmup)
|
10
|
8
|
10
|
?
|
Table One
Also worth noting is that the three coordinate sets obtained from the dedicated GPS devices were consistent with the overall accuracy level provided by the devices (falling with a circle with the location at the center and a radius of 15 feet). The locations obtained from the camera and the phone were much more variable with both devices providing some fixes that were hundreds of feet away from the actual locations (more on this later).
In addition to the latitude/longitude values representing the location, GPS devices also able to provide a measurement of the elevation relative to sea level. I recorded and averaged these elevations and Table One includes the comparisons of the calculated elevation with the elevation that Google Earth shows at that place. That reference elevation provided by Google Earth was compared with a nearby USGS benchmark and the benchmark value differed from the Google Earth value by just 2 feet.
The elevation measurements provide additional insights into the accuracy of the devices. The calculation of elevation using GPS requires that a signal be obtained from at least 3 satellites (as opposed to a minimum of two for the location only). Given that all of these devices were able to lock signals from 8 to 10 satellites I would expect the devices to provide accurate elevations, but, historically, elevation measurements made by GPS have tended to be less accurate and less reliable (for various reasons). The two GPS devices and the camera worked well under the ideal conditions of this testing, but the phone GPS was off by close to 100 feet on average.
Overall, the Garmin GPS performed best, but that is mostly a question of convenience. The Garmin is typically ready to go within 30-60 seconds and the Magellan unit commonly needs several minutes to lock signals from a full complement of satellites. The Garmin uses a far superior antenna technology but, this is no surprise, the Magellan unit is over 10 years old (a newer Magellan would probably be similar). Both GPS units and the camera provide geographic references that can used for a variety of purposes. The next question I want to consider is precision. What does it mean to say that a lat/lng pair are within 10 feet of the "actual" value. There's more to it than you might think.
In addition to the latitude/longitude values representing the location, GPS devices also able to provide a measurement of the elevation relative to sea level. I recorded and averaged these elevations and Table One includes the comparisons of the calculated elevation with the elevation that Google Earth shows at that place. That reference elevation provided by Google Earth was compared with a nearby USGS benchmark and the benchmark value differed from the Google Earth value by just 2 feet.
The elevation measurements provide additional insights into the accuracy of the devices. The calculation of elevation using GPS requires that a signal be obtained from at least 3 satellites (as opposed to a minimum of two for the location only). Given that all of these devices were able to lock signals from 8 to 10 satellites I would expect the devices to provide accurate elevations, but, historically, elevation measurements made by GPS have tended to be less accurate and less reliable (for various reasons). The two GPS devices and the camera worked well under the ideal conditions of this testing, but the phone GPS was off by close to 100 feet on average.
Overall, the Garmin GPS performed best, but that is mostly a question of convenience. The Garmin is typically ready to go within 30-60 seconds and the Magellan unit commonly needs several minutes to lock signals from a full complement of satellites. The Garmin uses a far superior antenna technology but, this is no surprise, the Magellan unit is over 10 years old (a newer Magellan would probably be similar). Both GPS units and the camera provide geographic references that can used for a variety of purposes. The next question I want to consider is precision. What does it mean to say that a lat/lng pair are within 10 feet of the "actual" value. There's more to it than you might think.
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