GPS

[John Mc]

How high up is your canopy?

I know a person who does a lot of GPS/GIS mapping here in Vermont's forests. I don't know what kind of GPS he has, but he does use an external antenna. He puts the antenna on the end of a telescoping walking stick. When he has trouble getting a lock, he can sometimes improve the situation by lifting up the antenna as high as he can on the end of the stick. It's funny because the stick only gives him an extra 8 feet max. This is no where near the canopy height, but it does seem to make a difference, and he doesn't have to climb a tree to make it work.

It might not work in your situation, but it might be worth trying a low-tech solution before looking at something more expensive.

[jim king]

John:

The canopy here is typically 80 to 120 feet.  It is dark inside the jungle.

 



 

[John Mc]

The canopy here is typically 80 to 120 feet.  It is dark inside the jungle.

A lot denser foliage than what we have in my area. Our canopy is probably 70 or 80 feet in mature hardwood stands. Still, it's tall enough that you wouldn't think lifting the GPS antenna up about 8 feet would make a difference, but it does. It may not make any difference at all in your forest, but it's cheap to try. I also wonder if there are aftermarket antennae which would help?

I have another friend who is an engineer who researched his GPS purchase in great depth before purchasing it. There is a huge difference in sensitivity between various models, and the most sensistive now beats what was commonly available 5+ years ago. He was looking for one which would lock on in our forests. I do know his works much better in the forest than my 6 or 7 year old Garmin aviation GPS.

John Mc

[Doug_D]

Jim,

I would start looking at RF differential navigation systems.  These are often used in maritime situations at a port of entry.  Google "loran" and see what you can dig up.

[jim king]

I just got this from the US forestry Service.  I guess it says it is not easy and a special type of GPS is required.

Hello everyone.

 
Using GPS under a tropical canopy is definitively a challenge.  That’s why I established a test network in such an environment to test GPS units.   But you need to keep in mind that not all tropical canopies are the same.  I have done extensive GPS/GIS work in Puerto Rico, Brazil, Nicaragua, Costa Rica, Mexico and once in Peru.   Let me explain a few concepts regarding accuracy with GPS before I continue.  I am limiting my discussion to Mapping Grade GPS.  You can only correct data for ionospheric delay of the GPS signal, GPS satellite ephemerides, and receiver clock errors, and before 2000 select availability.  User errors and multipath cannot be corrected.  Autonomous means that no correction is applied to the GPS data; Differential GPS (DGPS) means that you are applying some type of correction to your data, and there are several ways to do this:

·         WAAS (Wide Area Augmentation System)

·         Real Time

o   Satellite (commercially available)

o   Beacon (radio)

o   Virtual Reference System (VRS)

·         Post Process (PP)

The accuracy increase from Autonomous>WASS>RT Sat>RT Beacon>RT VRS>Post Process.  Post process will always be your most accurate data.  The best mapping grade GPS units claim PP accuracies of around 50 cm under ideal conditions, and RT corrections (except for VRS which is a new process) are typically 1.5-3 times higher than your PP accuracy, again under ideal conditions.  That said, you can start becoming skeptical of claims of sub meter RT accuracy under tropical canopies.    If you look at our test results (established by licensed surveyors), you will see that no PP result was sub meter.  Although new units have come out since then, I have had no time to go out and test them.  Looks like I will need to do so soon.  Recreational GPS units are not able to do Post Process DGPD, and rely on WASS for such corrections.  WAAS does not work under canopy (although it might get in rarely). 

As Chip well said, plot location is one thing, and stem mapping with that same accuracy is another thing.  In a research study we did in Mexico (3 sites) and Brazil (1 site) we established plots 500 meters by 500 meters (gridded in 100x100) to map mahogany trees growing in natural stands.  I used a laser ACE 300 with an integrated digital compass to lay out the grid and later geo-referenced the 4 corners with mapping GPS unit (therefore the whole grid is geo referenced).  Individual trees were mapped from any corner using the laser.   As you can see, a combination of techniques sometimes is the work around. 

Something I always do before going to the field (in tropical forests) is to use a GPS planning software to be in the field when the satellites are high in the sky (e.g. fewer obstructions).  The software allows you to put a curtain of obstructions and calculate optimal times in the day to work.  I usually use a 25° curtain all around.  You can download a free planning soft at http://www.trimble.com/planningsoftware_ts.asp   

Remember GPS is one more tool, and you need to see if it fits the purpose of the task.  Knowing the limitations and what to expect from the units will help you in their use.  Please do not hesitate to contact me directly if you need more help.

[woodtroll]

The way I take that, use a high end gps (trimble) and lay out some known points,then survey in to your trees. That sounds more difficult then gps'n the trees.
Could you send up climbers? They would not need to climb to the top but close. (They would also need to be crazy.) The Northwest has had climbers topping trees for years. It would be easier to take gps points then to saw the top off.

Are there any local base stations for gps correction?
What kind of accuracy are the regulators requiring?
Do you have anyone available to assist you with GIS, geo-referencing and mapping?

I regret the added regulations you are having to go through. It is very aggravating to see added rules and hoops to jump through, that sound good on the surface but do not make any sense.
I think you should have the USA gps every tree that will be harvested and exported, and while we are tracking information, gps all the seedlings planted also. Just to keep track.

[SwampDonkey]

Jim, I can see the price of your trees just jumped, oh say by 500 %.

"You want'm, show me the green and bring your GPS."

[jim king]

Are there any local base stations for gps correction?

No

What kind of accuracy are the regulators requiring?

They dont know

Do you have anyone available to assist you with GIS, geo-referencing and mapping?

Virtually impossible to find this type of person that can provide any thing you can believe in nor do they have equipment to do it correctly.  We have another problem here in that the local office of titleing has one set of maps and the main office in Lima has another set of maps that over ride the local office maps and neither are correct.  We bought a 7000 hect peice of land and had to send the maps from the local office here to Lima 7 times for them to agree.

We knew that after 7 times niether were correct as it included a village that we had nothing to do with but we `gave up and registered registered it anyway.  After it was regestered and both agencies were happy we brought in a crew to do it correctly , after a couple of weeks and several thousand dollars we reregistered the new map that was "more similar" to the land and slipped it into the public registers office as an updated  map and they accepted it.

They dont call this the third world trying to be fourth for nothing.

This is a real cluster

[Samuel]

Jim, are you having fun yet? 

I wish I had lots of time and money, as I would love to come down and spend some time getting to know your business and helping out where I could.

[jim king]

Jim, are you having fun yet? 

I wish I had lots of time and money, as I would love to come down and spend some time getting to know your business and helping out where I could.

Samuel:  We just had a visit from the US Justice Dept to explain to the people the penalties and problems if they do not comply with the new laws and this week is coming some other group from The US to find out why everyone is in noncompliance with the laws as dictated by the "Free Trade Agreement" with the US.

It is really a mess.  They are putting about 80,000 subsistance loggers out of business and forcing them into slash and burn for Cocaine.  It is a real mess made by real idiots.

[jim king]

The first positive step on the new forestry law is that we will now have a 50 meter variance on the GPS points of the trees and boundries to compenaste for the normal errors of a GPS in the jungle.  One problem down several more to go.

[Splinter]

Probably not much help to you, but what works for me in the northeast is a Garmin with an external antenna. It has a magnet mount and i screwed a piece of galvy onto an old cap and mount it up there when i think i need to know where i am. forgot it was there and stumbled on a hunter.. was wondering why he was looking at me funny till i remembered the funny hat with the wire on it.

found the stone corners the surveyors missed......
like this one
http://www.amazon.com/GARMIN-010-10702-00-GPS-Antenna-Kit/dp/B000BS2V80



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