This Month:
- The Coords are Off 23 Feet!
No wonder I can't find that blasted film can!By Rick Angelin, aka AB4N
- JWDATAMABy Tee King, aka Pyewacket
- Crossword Puzzle:I've Been Having Trouble
With My Garmin...By Tony Delgado, aka TableSaw
- Coining a Sig ItemBy The Gowen Family
The Coords are Off 23 Feet!
The aim of this article is to help the reader better understand the nature of GPS signals, and why experiences like (the coordinates are off 23 ft.) happen. Hopefully, this knowledge will enable you to be better prepared when hunting and hiding caches. When I first started geocaching, it wasn’t long before I had the desire to hide my first cache. I chose a spot I thought was interesting and few people, even those local to the area, knew about. I stocked an ammo can with inexpensive, but tradable, items, and then placed it where it was not likely to be discovered by muggles.
I took several coordinate readings at the spot, and quickly noticed that each reading was slightly different. So, I backed off and approached the spot from different directions, and took note of the various coordinate sets. I then averaged what I deemed the best numbers and used them when submitting the cache for approval.
Those who have hidden caches know that it is fun and interesting to read the logs as they arrive in your email inbox. But, it wasn’t too long before I got the occasional note that wasn’t so thrilling. "The coordinates are off by 23 feet," read one log. A log for another cache that I placed along a river bank read, "The coordinates put me out in the river, I wasn’t prepared for a swim."
If you have hunted for more than one cache, you have probably had a similar experience where, when you finally found the cache, your GPS receiver was showing that it was several feet away; maybe dozens of feet. The aim of this article is to help the reader better understand the nature of GPS signals, and why experiences like these happen. Hopefully, this knowledge will enable you to be better prepared when hunting and hiding caches.
GPS Signals
GPS satellites orbit above the earth approximately 12,000 miles. The satellite transmitter power is 50 watts or less! These signals are then received by your GPS receiver (GPSr) which calculates your location by "triangulation". Compare the signal strength of these satellite signals to your local radio station which may be 20 miles or less from your location and broadcasting at 100,000 watts!
Satellite signals will bounce off solid objects like buildings and rocks.This is why GPS receivers need line of sight reception, and obstructions such as hills, trees, rocks, buildings, etc. make reception more difficult. If your GPSr is WAAS enabled, and you are receiving the WAAS signals, your GPSr will be able to calculate your location more precisely.
Satellite signals will bounce off solid objects like buildings and rocks. The satellites and your GPSr do not know objects like these are present; your GPSr is expecting a straight line of sight signal from the satellite. If you are in a city with buildings around and asphalt or concrete under your feet, or if you are in mountains surrounded by rocks and trees, imagine what your GPSr is having to do in order to calculate your location with all those signals bouncing around!
GPS Receivers
As technology has improved, modern GPS receivers are more capable of deciphering the signals and providing coordinates that are relatively accurate. However, the less ideal the situation that you’re in, the greater the error will be for the coordinates that your GPSr indicates. Modern consumer grade GPS units are accurate to about ten feet under ideal circumstances.
Most GPS receivers have the capability to display the estimated error. This number is a radius from the actual location of the coordinates which could be anywhere inside that circle (See figure 1).
Figure 1
Now, consider that the person who placed the cache may have been standing somewhere outside this circle, but within 23 feet of the red X (see figure 2), presuming their estimated error is also 23 feet.
Figure 2
The circle on the left represents the area that your GPSr indicates as a search area, and the circle on the right represents the area for the GPSr of the person who hid the cache. The circle on the right might be positioned anywhere around the other circle, potentially extending the radius to 46 feet, a search area of 6647.6 square feet, or roughly the area of two large houses!
Of course, this example uses the worse case scenario for a 23 foot estimated error on the GPS signals, but serves to show how the error of two different GPSr will increase the area that will need to be searched. How many hiding places might there be in any given 6000 square feet area?
Coordinates
Coordinates are shown in a variety of formats. The same spot on a map might be indicated as:
- DEG° MIN.MM
- DEG° MIN.MMM
- DEG° MIN' SEC''
- DEG.DDDDD
- UTM
In addition to the various coordinate formats are various datum. It is not within the scope of this article to elaborate on these various formats and datum. Suffice it to say that WGS84 is the datum commonly used in geocaching, and DEG° MM.MMM the format for the coordinates.
To put these numbers into perspective, let us use the following set of coordinates for example: N35° 12.345 W085° 45.678
How far off would we be if the least significant number of the longitude were off by one? For example: N35° 12.346 W085° 45.678
Answer: 6.08 feet
A single number difference in the latitude coordinate is equivalent to approximately 5 feet. A single number difference in the longitude coordinate is equivalent to approximately 6 feet. If the latitude coordinate were off by .002 and the longitude off by .003 the error in the coordinate set would be 20.78 feet. The following chart illustrates the distance of error in feet if the coordinates are off from .001 to .009
| Latitude | .001 | .002 | .003 | .004 | .005 | .006 | .007 | .008 | .009 | |
| Longitude | 0 | 4.98 | 9.96 | 14.94 | 19.91 | 24.89 | 29.87 | 34.85 | 39.83 | 44.81 |
| .001 | 6.08 | 7.86 | 11.67 | 16.13 | 20.82 | 25.62 | 30.48 | 35.38 | 40.29 | 45.22 |
| .002 | 12.16 | 13.14 | 15.72 | 19.26 | 23.33 | 27.70 | 32.25 | 36.91 | 41.64 | 46.43 |
| .003 | 18.24 | 18.91 | 20.78 | 23.57 | 27.00 | 30.86 | 35.00 | 39.33 | 43.81 | 48.38 |
| .004 | 24.32 | 24.82 | 26.28 | 28.54 | 31.43 | 34.80 | 38.52 | 42.50 | 46.67 | 50.98 |
| .005 | 30.40 | 30.80 | 31.99 | 33.87 | 36.34 | 39.29 | 42.62 | 46.24 | 50.10 | 54.14 |
| .006 | 36.48 | 36.81 | 37.81 | 39.42 | 41.56 | 44.16 | 47.15 | 50.45 | 54.01 | 57.78 |
| .007 | 42.55 | 42.85 | 43.70 | 45.10 | 46.98 | 49.30 | 51.99 | 55.00 | 58.29 | 61.79 |
| .008 | 48.63 | 48.89 | 49.64 | 50.88 | 52.55 | 54.63 | 57.08 | 59.83 | 62.86 | 66.13 |
| .009 | 54.71 | 54.94 | 55.61 | 56.72 | 58.22 | 60.11 | 62.34 | 64.87 | 67.67 | 70.72 |
From this chart, you can see how a small difference in the numbers can make a big difference in search area. If you ever have a chance, hold two GPS receivers side by side and see if you get the same numbers. Often you won't; this is due to the terrain in conjunction with the various makes and models of receivers. GPS units are radios, and the quality and technology used make a difference in how you see the coordinates.
In Conclusion When looking for a cache, or when hiding one, the best method is to approach the target area from several different directions if possible. Hopefully, you have learned that caches will sometimes be located several feet, even dozens of feet, from where your GPSr points you. Experience will often be necessary to find that elusive cache. Of course, a few geocachers like to fool those of us who might think we know what we are looking for and where to find it ;-)
When looking for a cache, or when hiding one, the best method is to approach the target area from several different directions if possible. From these approaches, the likely hiding area can be more accurately identified. You may also need to stand still and let the GPSr average the signals for a while. Often this averaging will change the initial coordinate reading by 20 to 70 or more feet. This being the case, it is especially important that, when hiding a cache, several readings be taken by using the multiple path approach method in conjunction with letting each reading average for a period of time. With a little care, you can minimize the logs that read, "The coords are off 23 feet!"