Repeatability of GPS under clear conditions, and under a canopy.

I performed a simple back-yard experiment with my Leica 530 GPS system this morning. I set the base station in a clear area by the pool, set a point nearby on the deck, and a second point under a tree (Acacia) canopy. Every 20 minutes or so I shot the points. Each was measured once, then shot five times in a row using the Leica averaging feature. For the mean coordinates I used the average of the multiplied points.

I wanted to see how the Leica 530 would measure under normal field conditions, including how well I held the rod on the point using a bipod. Satellites ranged from 8 to 10 to 8 during the five hours I ran the experiment.

In the last hour I could not get any shots under the tree canopy, but got them easily on the deck. In the real world I would simply have extended the slide pole to get above the canopy, but in this case I wanted to see how the unit would measure under the canopy.

Note the one large elevation deviation on a single tree shot. Although I had the position limits set for 0.05' H and 0.06' V, with a combined quality of 0.08', the system did not detect the bad shot under the canopy.

The results indicate to me that the Leica 530 is quite suitable for boundary and topographical work, but care must be taken near a canopy because degradation does occur (which is why I use a slide pole, and have a variety of offset routines).

Always be aware of the possibility of a bad shot from a variety of multipath sources, including trees, power poles, and building walls. I have detected five bad shots on my surveys in the past several years and every one of them was unexpected.

Bruce F. Small

Tucson, Arizona

August 26, 2001

Multiple deck shots Single deck shots

H V H V

0.01 +0.05 0.01 +0.06

0.01 +0.05 0.00 -0.03

0.01 -0.03 0.03 +0.02

0.03 +0.01 0.01 -0.01

0.01 -0.00 0.02 +0.03

0.01 +0.02 0.01 -0.06

0.01 -0.03 0.01 -0.05

0.01 -0.03 0.01 -0.04

0.01 -0.03 0.05 +0.01

0.03 +0.01 0.03 +0.03

0.02 +0.03 0.03 +0.00

0.02 -0.00 0.01 -0.03

0.01 -0.04

Multiple tree shots Single tree shots

H V H V

0.04 +0.03 0.03 +0.03

0.04 -0.05 0.06 -0.08

0.01 -0.01 0.00 -0.04

0.05 +0.02 0.05 +0.02

0.06 -0.07 0.07 -0.07

0.06 +0.09 0.07 +0.18

0.01 -0.03 0.02 +0.04

0.03 -0.03 0.02 -0.04

0.01 +0.05 0.03 +0.07

0.03 -0.04