5 replies to this topic

[Charles Syrett]

I'm looking into the possibility of getting a Magellan MobileMapper with post-processing, which allegedly yields sub-foot accuracy. I've never actually done post-processing before, and there's a high price tag on this, so I'm a little nervous. One shop I called was really trying to sell me two units, since you "have to" have two units to make post-processing work. When I told him that I assumed I would be using data from beacons, he implied that there are very few of those, you have to be within 20 km of them, they're not reliable anyway, etc. etc. I also asked about "high performance in dense canopy cover" (as described by Magellan), and he all but denied the truth of this.

Does anyone have any experience with post-processing? Even the sub-meter accuracy sounds good, but sub-foot makes my mouth water.

Charles Syrett
Map Graphics
http://www.mapgraphics.com

[Eric Wolf]

Charles,

I've used a Trimble GeoXT and a Pathfinder for DGPS. The accuracy is nice - but sub-meter takes time. Basically the unit has to remain stationary and record all the satellite signals over a period of time. In post-processing against a base-station, you basically eliminate error created by the environment by subtracting error in the same signals received at the same time by the fixed-position base-station. Of course, that assumes the environmental conditions at the base-station are the same as they are/were for your receiver.

An RTK unit can receive signals from an RTK "beacon" and make these corrections on the fly. Of course, you are limited to the range of the beacon and differences in environment between the source and your receiver. In the US, the NGS provides a map of both commerical and co-operative Continuously Operating Reference Stations (CORS) here: http://www.ngs.noaa.gov/CORS/

There may be a Canadian analogue.

So another solution is to use two units. You setup one in advance of your survey and let it average it's location over a very long time. Then you use it as the base station to correct against. If your survey is near an established survey monument, you can even setup your temporary base on the monument and not have to average over a long period of time.

Tree canopy is a big problem because GPS works on line-of-sight microwave signals. Interrupting the line-of-sight with the satellites introduces errors. There's really no way around it short of surveying techniques.

Overall, what the salesman told you was dead-on.

I used to have a Garmin GPS V - a simple consumer unit - that was able to average it's location over a period of time. I never tested to accuracy but it claimed to get down to about a foot with an hour of averaging.

[Charles Syrett]

Thanks, Eric. Does the second (stationary) unit have to be just as fancy and expensive as the mobile unit? Or can I get away with buying one MobileMapper and one consumer Garmin?

Charles Syrett
Map Graphics
http://www.mapgraphics.com

Charles,

I've used a Trimble GeoXT and a Pathfinder for DGPS. The accuracy is nice - but sub-meter takes time. Basically the unit has to remain stationary and record all the satellite signals over a period of time. In post-processing against a base-station, you basically eliminate error created by the environment by subtracting error in the same signals received at the same time by the fixed-position base-station. Of course, that assumes the environmental conditions at the base-station are the same as they are/were for your receiver.

An RTK unit can receive signals from an RTK "beacon" and make these corrections on the fly. Of course, you are limited to the range of the beacon and differences in environment between the source and your receiver. In the US, the NGS provides a map of both commerical and co-operative Continuously Operating Reference Stations (CORS) here: http://www.ngs.noaa.gov/CORS/

There may be a Canadian analogue.

So another solution is to use two units. You setup one in advance of your survey and let it average it's location over a very long time. Then you use it as the base station to correct against. If your survey is near an established survey monument, you can even setup your temporary base on the monument and not have to average over a long period of time.

Tree canopy is a big problem because GPS works on line-of-sight microwave signals. Interrupting the line-of-sight with the satellites introduces errors. There's really no way around it short of surveying techniques.

Overall, what the salesman told you was dead-on.

I used to have a Garmin GPS V - a simple consumer unit - that was able to average it's location over a period of time. I never tested to accuracy but it claimed to get down to about a foot with an hour of averaging.

[chris thompson]

At our company we use Trimble GeoXH gps receivers with the 'Zephyr' antennae model and use post-processing to differentially correct those data - the resulting accuracy is typically claimed to be in the 15-30 cm range; however, strenuous testing of this would probably show this to be closer to the 30-45 cm range.

A two unit system that you suggest sounds like it is a typical RTK (Real-time kinematic) and would include a base station that you set up over some known control point and a rover unit that you would carry around to collect your data points. In such a system the base and rover units are in constant communication with one another, the base providing corrections to your incoming gps positions you are receiving on the rover. This is the system that land surveyors use and done correctly (its more complicated than simply setting the equipment up) results in accuracy that can be measured in fractions of an inch.

It may be that the Magellen will allow some sort of base/rover set up that will allow you to use the base data as a correction source for post-processing - I don't know much about this system. However, the Trimble system allows you to download base data as a correction source from any of the CORS (continuously operating reference stations) stations and I'd think that the Magellen system should allow this too - surveyors using RTK or even static GPS use CORS data as a correction source.

Also, while sub-foot accuracy may make your mouth water, you really need to ask yourself if the data you are collecting requires such accuracy levels and warrant that additional cost burden - not only is the equipment and software more expensive up front, the added costs of developing data standards, field collection methods, and post-processing and post-collection management are higher. Keep in mind that unless you are making maps at relatively small scales you'll never be able to distinguish between WAAS DGPS corrected and post-processed corrected data.

If you're intent on buying into a sub-foot accuracy system some things you should ask your supplier include:
1) Can your rover data can be post-processed using CORS data and if the software that you will need to use will automatically seek out those data from CORS stations or if you'll need to manually download data files. If you absolutely have to have a 2nd base station then you need to consider the risk of leaving your 2nd unit operating out of sight, or the cost of having a 'babysitter' watching the unit to cirucumvent theft or vandalism, and the cost of maintaining a 2nd unit.
2) Ask them to demonstrate the accuracy levels under real-world operating conditions and to disclose the data collection conditions that their glossy literature cite. So often the accuracy statments made by vendors are based on massive quanties of data collected under ideal conditions which your field staff rarely (if ever) encounter.
3) If you use GIS software, does Magellen data integrate directly into whatever data system you have (such as ESRI's Geodatabase). Find out what you'll need to do to get data from the data collector into a form that is suitable for making maps from. You may find that the cost of these additional steps/procudures doesn't warrant the additional cost of equipment and software.
4) Ask if you you need to buy an expensive external antennae or other equipment in order to post-process your data.

Good luck!!
Chris Thompson

[Charles Syrett]

Chris, thanks for reviving this thread. I did end up buying the Magellan, and I'm pretty happy with it -- it does do post-processing, and it works very nicely in areas of heavy canopy. There are some good online forums for this instrument (with mixed reviews!) to help newbies like me along. However, it's just a tool, and like any tool you have to go up a learning curve to get the results you want. Sub-foot accuracy is no more guaranteed than a flawless playing of the Moonlight Sonata on a new piano!

Charles Syrett
Map Graphics
http://www.mapgraphics.com

[ghopdata]

I have been using what was then the 'Thales' MobileMapper with a beacon for a couple of years. Not post-processing but RTK using the DGPS signal ftrom a Sydney [Australia] lighthouse and when south a Melbourne Lighthouse with good results but believe the DGPS has lowered the accuracy from +/- 10m (without DGPS) to +/- 3 metres.
I can receive the signal up to 50kms from the lighthouse, but in both locations there are no mountains between me and the coast.
The accuracy of +/- 3m is all I want - mapping mountain bike trails - and the data dictionary and MM Office software are good to work with.
Hope this may be helpful!
Peter