A question about distance accuracy on GPSr

[+rovers3]

Here's the scenario that brought about this question.

Yesterday I did an earthcache at a bog and one of the requirements was that you needed to measure the distance between two locations, numbered posts, using your GPSr. Well we ( I ) forgot about that requirement during the walk so I had to backtrack to get the measurement and I had the opportunity to measure it twice, once going westwardly and the other going in the opposite direction back to my waiting wife.

It was a cool drizzly day and I had 10 sats no waas with a Garmin Oregon 400t, latest upgrade, showing 4m accuracy, walking on a boardwalk over the bog. In the westwardly direction the measurement was 326m and on the return trip the measurement was 308m. I consider this a large difference over such a short distance, remember, I was on a boardwalk in an open area.

So I wonder how accurate measurements over large distances would be seeing how some of the trail organizations are now measuring the trail distances using GPSr's.

[+StarBrand]

How did you measure the distance?

 

I would have got a nice averaged reading at both ends (assuming it is in a straight line). Figure the distance between the points.

 

I am guessing you walked it?? and then maybe looked at the trip screen?. Your walking may not have been terribly consistent. The Oregon and several other units are notoriously bad at tracking people well as they walk along slowly. If you load your recorded track up to a PC - I'll bet you get yet another measurement different from your other 2.

 

The units are accurate (within about 15 to 25 feet avg) of any given point but not so good at a highly accurate measure of distance while moving slowly.

[planewood]

Here's the scenario that brought about this question.

Yesterday I did an earthcache at a bog and one of the requirements was that you needed to measure the distance between two locations, numbered posts, using your GPSr. Well we ( I ) forgot about that requirement during the walk so I had to backtrack to get the measurement and I had the opportunity to measure it twice, once going westwardly and the other going in the opposite direction back to my waiting wife.

It was a cool drizzly day and I had 10 sats no waas with a Garmin Oregon 400t, latest upgrade, showing 4m accuracy, walking on a boardwalk over the bog. In the westwardly direction the measurement was 326m and on the return trip the measurement was 308m. I consider this a large difference over such a short distance, remember, I was on a boardwalk in an open area.

So I wonder how accurate measurements over large distances would be seeing how some of the trail organizations are now measuring the trail distances using GPSr's.

 

The Oregon does not have an antenna connector port but you'd be suprised at how much better accuracy you can get with an antenna.

 

Try this: Find a spot nearby your house with no overhead obstructions and start marking that location. Mark it then let it average for about 15 seconds then record it. Repeat that about 30 times at that same spot. (or turn tracking on for 30 minutes) Then upload those locations to Google Earth and look at the points. You'll probably see them arranged in a smooth curving line.

 

Why aren't all the points exactly the same? Cause the satellite configuration is constantly changing and the signal strength the unit is receiving is changing. (an antenna will greatly help with the latter)

 

There generally are about 2 times during the daylight hours that the satellite configuration is optimum. Trimble has a program (free) on their website that will plot those times. Taking measurements during the optimum times can give much better accuracy than during the least most optimum times. (if you going out geocashing on a particular day then make an optimum time chart and go at the best times)

 

I have a spot out in the center of our cul-de-sac that has a very accurate lat/lon survey. With my GPSMAP 60cx I can get 5' repeatable accuracy with an antenna under clear, cool, low humidity skys and with the readings taken during the time of optimum satellite configuration. For a $300 hand held unit that is pretty darn good!

 

Without an antenna and during least most optimum times the average accuracy is about 40' - 50' at this spot.