The Thales ftp Application Note for requirements on postprocessing accuracy calls for a 20-minute track on 5 sats/<4 pdop before collecting data that may equal submeter. I've only been collecting point data thus far and have waited for WAAS lock (5sats/<4pdop) before logging a 20-second point feature. These data (wildlife, etc.) aren't submeter requirements, but I want to eventually attempt a wetland line in which case I'd like close to a meter (it will strictly be for my own project use, not for jurisdictional/agency needs). Therefore, will I need to obtain 20-minutes of logging before collecting each wetland line to have a shot at 1-meter, or is it a one-time deal to acquire 20 minutes worth so that MM has enough to accurately average in that locale? When I first got the MM I was logging 60 seconds for a point, but then heard that 20 seconds would be fine for 2-3 meters.

As a sidenote, these application notes off ftp site are very informative, especially for non-surveyors like myself and I can see that lots of time went into those. I'm also enjoying the posts on this site.

Tom

I'm curious.

I don't do wetlands, but I've always wondered if elevation ever becomes a factor in doing these-type jobs.

Does a surveyor just go out and locate the edges of a wetland, or is the wetland previously marked by a wetlands "expert", and the surveyor is only denoting those parameters?

Tom,

You don't need to wait 20 minutes before logging points. Twenty minutes is recommended for the entire session -- this gives enough time for the satellites move and cause a shift in their gometry.

Linda

Please don't take this as a contradiction of what Linda just said because what she said is basically correct. Twenty minutes is a pretty good number to use as a minimum session length.

I have found that the longer I go with a good solution running the more accurate the results of my data capture. By idling for 20 minutes prior to my first shot, and doing the same after my last shot, while doing my best to maintain a solution as I go along, I have been able to do much better than sub-meter accuracy.

If I walk under a nice shade tree, or get in the shadow of a tall building, that period of time that it takes to get back to those best quality solutions is about 15 to 20 minutes.

What I have noticed is that I do better to let it idle and then take a shot rather than to take a long shot on a feature. So if I come out from under a bridge I just walk around in the sunshine for a little while and then walk over to my next feature and take the shot.

But this is just what I have seen with some of my little science projects.

John,

We (consultants/engineering companies w/o survey capability) always have the survey company whose already doing our topo, right-of-way, etc. capture our flagged wetlands. For instance, I'm just wrapping up a wetland survey of a 6-mile road improvement. I flag and label the wetlands and each flag point (WL-1E.1, WL-1E.2, etc.) and the surveyors capture those polygons and they are generated as part of the survey files and then sent to my company (where we are doing the improvement designs, drainage, environmental permitting (me), etc.).

So, yes, the surveyor is coming in and capturing/denoting those points and since they are survey-grade our engineers have no problem w/signing-sealing those wetland files. I'm actually a certified Professional Wetland Scientist (PWS) (easy when you grow up in Florida!), so I have to be able to accurately/consistently flag those features (marshes, forests, many ditches, etc.).

There's many reasons, as you know, why it would make sense for me to manage my part of the fieldwork w/unit like MM. For instance, I currently place a dot on a large-scale aerial and label it for each wetland point. This job I'm currently working on has about 40 wetland features so we're looking at around maybe 300 or so of those dots (flags) on aerials. But I'm used to it so I get them on and neatly, then pass on to surveyors in case they can't find some of my flags. The reason for my interest in wetland GPS is so I can capture/manage info more efficiently (I think, but not sure yet if more efficient).

Wow, getting carried away here. OK, thanks for question.

Tom

I suppose you are aware that you can take the surveyor's cad files and create either a shape file or a dxf file, upload it to the MM and do a whole lot of real time retracement.

We work in Texas SPC's and very often take cad drawings we created from georeferenced DOQQ's, or other sources of georeferenced aerials, and load them as shape files to the MM. The very best recon tool we have ever used.

Really nice tool for post processed positioning as well, especially if you can operate your own base station within a mile or two of the project site. The CORS ties are great for general submeter work, but the local base gives a real advantage.

JD

You do not have to have 20 minutes on a point to get very good results for a MM point.

People are mixing up PM's and MM's...

I like to let the unit cook while on the way to the job..I want it to get a good solid almanac..That's about all.

I have done thousands of points using 1-3 minutes and have got results in the sub-foot range when doing post processing.


Submeter processing is just like DGPS execpt after the fact...Not in real time.

I have had killer results with 10 seconds worth of data.

Deral

Deral, please don't confuse what I said to mean you need 20 minutes on a point. What I said was that the longer I can maintain a solution the better the results seem to be. Maintaining the solution does not require sitting still on each point. It does require some effort to use the system in a way that keeps data flowing to the GPS receiver.

But those are just my little science projects and my personal observations. Like you, I see very good results with a few seconds of data on each point. It is just that the results seem better if I have about twenty minutes of solid data prior to that few seconds on that point.

But I am content for any and all to disagree. My comments are about what I have seen and do not represent what I would describe as official answers.

By doing what I have done I have seen repeatable results at sub-foot.

Great comments above and very informative for me......and it all makes sense to boot. It appears as though my usage for points thus far is along these lines. I do like the 20-minute cooking/solution ideas while en route or initial time at locale. I've also noticed that seems to make a difference in keeping a WAAS lock as I walk from one feature to another.

JD, great idea re the use of surveyor's files for recon and positioning. I wasn't aware of that, but I'm usually finished in one project area by the time surveyor's files are delivered to us. However, there are times when I know survey files exist for area where I'm going to work (e.g., former wetlands jurisdictional survey lines). Sounds like a great exercise though.

Tom