TOPIC:

I have a question about centimeter accuracy on gps units. Ive noticed most survey companies use antennas with thier gps units. We are using a centimeter enabled Geo Explorer 7x Series. Is there any difference in the accuracy? What about in tree cover?

The external antenna is recommended to reliably achieve centimeter accuracy / the maximum possible accuracy of the unit. The Zephyr model 2 (external) antenna provides a 50 dB signal gain for reliable tracking in difficult environments such as tree cover or near buildings. By using an external antenna you will then have the antenna mounted to a fixed height range pole which will assist in a more reliable elevation additionally you will level the antenna directly over the location you are collecting making for a more accurate XY. In a nut shell you can collect accurate data without the external antenna however by having one you increase the signal strength of the satellites which improves accuracy. And by collecting data with a range pole you force yourself to have the antenna precisely over the point (leveled) and the antenna is at a fixed height both of which improve accuracy. Since surveyors are required to be as accurate as possible at all times this is why you always see them with an external antenna attached to the receivers.

That answers my question perfectly, thank you!

Thats great information about the antenna, our antenna has been in the corner of the office but now will start using our antenna. Thank you.

I use the Trimble GeoXH 2008 series with a Tornado antenna. I played around with my GPS setup a few years ago and collected a known point location with the integrated GEO antenna and then with the external Tornado antenna. I had clear view of the sky and was able to maintain a lock on satellites, which enabled me to take advantage and track both carrier signals (L1 & L2). My results after 3 locations showed an average of 7.54 cm difference between the antenna's. This was after correction with Post Processed Carrier Float type. This may not seem like a huge difference, but again I was collecting under open sky. I have found the antenna makes the biggest difference under canopy and tougher environmental situations. I imagine the new Trimble series Geo's with GLONASS are phenomenal! Don't forget that your differential correction and transformation processes are just as important for map accuracy as using the antenna! Have fun!