We are non profit and are looking for a GPS device that we can put against a tree to get the GPS coordinates withing one [1] meter. We don't need it finer than one meter. If it is finer than great, but we do not want to pay for accuracy that we don't require.
From what I have been reading, WAAS will get us within 3 meter and L2 will get us within 1 meter.
Being a non profit we can't spend $1000 of dollars on units. Does anyone have any recommendations on units and if they can be rented?
Thanks in advance for any advice. Paul W
GPS receivers fall into 3 broad categories. Recreational grade and car units will not come close to your spec. Resource mapping grade in in that general accuracy range and perhaps marginally affordable for you, but I'm not familiar with their specs in any detail. Survey grade is much too expensive for you.
But be aware than no unit will reliably give its specified accuracy under tree canopy.
> We are non profit and are looking for a GPS device that we can put against a tree to get the GPS coordinates withing one [1] meter. We don't need it finer than one meter.
Just out of curiosity, what is the intended use of the coordinates of these trees? Are you trying to determine which are on which side of a land boundary, or is this just for laying out roads and trails?
Why an uncertainty of +/- 1m? Why not +/-5m?
When used correctly on a good mapping grade receiver, WAAS will get you sub-meter horizontally. However, as has been noted, no receiver will perform to its specifications up against a tree.
If renting is an option for you I'd suggest contacting your local Trimble dealer and renting a Geo. They come spec'ed at different accuracy levels and there are various techniques you can use to achieve those accuracies - WAAS, post processing, RTN, etc. If you have a good dealer they'll be willing and able to advise you on the best way to achieve what you're after.
GPS. And Trees. Yes. That is a problem. Think offsets. Or, think problems.
N
Your best bet would be to find a surveyor allied with your Non-Profit who would be willing to help out. Especially if this tree location is in relation to a property boundary (otherwise why need it within a meter?), you will be wasting your time and money learning about and renting equipment to give you results that you can neither rely on nor use in any legal sense.
GPS signals are modulated on a very weak frequency and any interruption, eg. trees, will quickly degrade your solution to being nearly meaningless.
Andy
Maybe post about your non-profit, where you are, and why you need this tree located and someone here will pop up to help you out?
A huge difference in results can result based on the type of tree and time of year. Knowledge of the best orientation to have a view of the most satellites is important, too. Being on the north side of a cedar/juniper tree may not work at all, let alone give you terrible numbers. Also, the number of trees blocking the satellite view is critical. A lone dead tree in the middle of an open pasture is almost no problem.
High quality mapping grade receivers like the Geo don't have anywhere near as much problems in canopy as survey grade receivers.
> Being a non profit we can't spend $1000 of dollars on units.
Contact your local Leica and Trimble dealers to discuss GIS type resource grade collectors. You will be looking at spending $5k - $10k per collector, there will be some software as well, training involved, etc. They can be rented, of course.
These dealers and your local municipal GIS departments will know who is using these types of units in your area. If you could identify a local surveyor knowledgeable in these particular units who can advise you perhaps some consulting arrangement could be worked out.
Putting the collector up against a tree will block out most of the satellites and seriously degrade the positional quality. Generally your procedure will be to collect points some distance from the tree, with a bearing and distance offset to the tree. The quality with which you collect these bearings and distances will have much to do with the final quality of your tree positions. If this is fairly heavy woodland or forest you may have problems using GPS at all.
You will get positions, but just because the brochure says that the collector is capable of getting +/- 1 m positions doesn't mean that all, or even most, of the positions it gets are that good. Equipment salesmen rarely stress that.
If your 1m positional quality is hard and fast you may be looking at hiring a surveyor to meet that spec. Good Luck.
Paul,
Please do not take what I am about to say the wrong way. I mean this not as a personal indictment of you but of a general line of thinking that is a big part of what ails our profession today.
What you are asking is for a tool when what is truly needed is a solution. A tool, any tool, improperly used will not provide the results you need. Indeed, with many professional tools, they are subject to tremendous misuse and abuse.
Many in the public think that having the tools we use will infer upon them the ability to get the appropriate solution. While it can happen, you would be surprised at how often our clients need help at first identifying the appropriate questions to be asking.
Is there a tool that will magically fulfill your need? Maybe. But probably not unless the proper question is first identified and the appropriate level of skill is used with the tool.
Boil it all down and you get this advice. Seek a local licensed professional to help you. When you go to a doctor you do not start telling him or her what tools to use to help you get better. Neither should you prejudge what might be needed by a professional surveyor who can help you with your situation.
Larry P
I have noticed this as well. The Geo series can pretty reliably give between foot and decimeter in heavy foliage, while survey grade receivers seem to be centimeter or feet. The obvious answer is because the positioning algorithms used have entirely different goals.
A used Trimble Geo is still well over $1,000.
Grasshopper wants advice, ok. I'd be going with Larry P on this one. The best tools in unskilled and untrained people will render very doubtful results, which at times can turn out almost catastrophic. With all due respect.
And as noted above, what are in intentions of this sub-meter accuracy topo data and who is signing off on it? Heck, most of us could get sub-meter with a compass, quad map, hand held $150 Garmin, and/or a hip chain. Lots of flagging to stay on line is helpful too, just remember to turn around and have a look see at where you've been. Toss in some highly calibrated pacing skills and you're good to go.
Seriously though, I was involved in the early stages of a city GIS project. They settled on a Trimble something GIS grade receiver alleged to get within a foot after post processing. I didn't have permission to mess with it, but it's supposed to work after you download CORS data for the PP. But they did get a meter or so pretty reliably. I think they were around $4K or so.
RE: against a tree. Don't do that. Get an offset tie or just group them together. Me, I'd just use my handy dandy Garmin and round to the nearest 10 ft. BTW, any error in those snap shot ties are not consistent. Some could be 3 ft North and the next one is 6 ft South. There is your 9 ft you're looking for.
> High quality mapping grade receivers like the Geo don't have anywhere near as much problems in canopy as survey grade receivers.
Is there a published paper somewhere to verify that claim?
Are we comparing apples to apples in data?
Seems to me that the difference might be that the positional standards of "survey grade" instruments
Are more strict than mapping grade results.
I could accept all kinds of crappy solutions from my RTK in the woods, but it wouldn't mean the
Locations meant a darn thing.