GPS

[jim king]

Does anyone know of a GPS that can penetrate the forest canopy without cutting down a pouple of trees to get the coordinates ?¿

[BaldBob]

While some of the more expensive GPS's may be better at receiving signal under a heavy canopy, none are especially reliable under such conditions. The only reliable way to get the coordinates of a location under heavy canopy (short of felling trees to open the canopy) is to get the coordinates of the nearest location with a clear line of sight to the satellites, and measure the bearing and distance to the desired location. It is then a simple math problem to calculate the coordinates of the desired location.

[bill m]

I have both a Magellan Triton 1500 and a Garmin Rino 530 HCx and have no trouble receiving a signal out in the woods here in New England.

[Jeff]

I have GPS nav in my phone, and I also have a garmon. Neither will penetrate the summer canopy of a hard maple stands, or Northern White cedar in the U.P. , I have to find clearings to get them to work correctly. I see no way it would work adequately on the floor of your rain forests.

[jim king]

Jeff:

I have a drawer full of GPS´s and not one will penetrate the canopy.  As you said we have to look for the nearest clear spot, climb a tree or make a clear spot.

[woodtroll]

We use Garmin, DeLORME and Trimble units. The DeLORME and Garmin work as well or better for general use then the Trimble unit.
As units and companies update their equipment, the chip sets and receivers get faster and better. A unit from 3 years ago is not as good as one today. They also update the programing.
Check into DeLORME's  PN series. It allows you to use other map data, and has forms option that may help your data collection.
I use them for wildfire fighting and like them.

[SwampDonkey]

I use a Trimble GEOXM for cruising under canopy.  It's a 1-3 m GPS, whereas the older GEO3 was 3-5 m. There are two more handhelds up the line the XH and XT with increasing accuracy and H-Star real time correction on the spot. Another thing is to set up a base station in your jungle in a clearing and use one of these handhelds as a Rover. Now your talking $$.  ;) Surveyors do this. With the XM I will not get a continuous lock on a position, there will be multipath. But, I always figured if you stand at your position for a few seconds it's close enough to the spot. I have aerial photography on the screen and can verify that I'm near a creek, a road or a loan big old pine tree. I always average my vertexes when traversing and then base correct using government base stations. When averaging, I find the base corrections have pretty much nil influence.

I think the forest company puts a sampling layer onto Garmin units and sends fellows out to do their densities on those grids. They give the boss a layer for his Garmin Map76 to locate harvest blocks.

[Clark]

I have GPS nav in my phone, and I also have a garmon. Neither will penetrate the summer canopy of a hard maple stands, or Northern White cedar in the U.P...

I've honestly had very little to zero problems with a GPS getting and maintaining signal under such conditions.  Right now I use a Garmin eTrex Legend, pretty bottom of the line as far as GPS units are concerned. 

If I've ever had consistent problems its been while under red pine.  For some reason accuracy tends to degrade for me whenever I'm under red pine.  Ponderosa pine isn't a problem but red is which makes no sense to me as the two are morphologically very similar.

One problem that tends to be in a constant cycle for me is the number of satellites available.  Usually once a day I tend to get very few satellites available and a decent signal difficult to lock onto.  That period of minimal satellite coverage tends to come the same time the next day or slip one way or the other by an hour or two over time.  I don't know if that is the way the satellites' orbits works out or if it something that I have gathered without testing.

Clark

[jim king]

Clark:

I have a Garmin eTrex also as well as others and they just cannot get thru the canopy.  All are 3 to 5 years old so maybe the technology has improved.

[SwampDonkey]

With Trimble's, or others if you prefer, mission planning software you can see when your best coverage is for your area. We usually have a time between 10:30 am and 1:30 pm where the PDOP can be high with too few satellites or many satellites too low on the horizon as well as too many directly over head giving poor geometry. You can set your min PDOP level and other things to filter out the bad positions in a Trimble handheld and other higher end GPS. An antenna may help you some Jim. I'm under canopy all day in thickets, but up to 30 feet tall. But, still thick and when on the edge of the big woods, that tree height goes up to 80 feet. Sometimes you have to wait a few moments to get your points averaged around noon as mentioned. It doesn't last a full two hours for the low coverage, maybe 30 minutes where you have some trouble gathering vertexes.

I trust a Garmin for the area it maps, but not for the location it puts you. I've seen them off by 100 meters from where the traverse should be. I checked one block against my Trimble , loaded property maps and ground evidence with painted trees and the cut was to the line trees. Also, with aerial photo showing my cursor where I should be on the spotted tree line on the ground and indicated cut edge on the photo. And the front of the traverse followed the main road where it was suppose to. The Garmin had the new traverse over on the adjacent property. ;) If your doing areas or positions with a Garmin, make sure you get a 3D signal on the screen at each vertex or point.

Measured a thinning block one time and paid the guy. He said he got paid for half. I said the boys didn't finish the job. Boy was he mad. I guess he thought I was going to pay him an advance or something. Nope!!!  Actually, come to find out, his guys lied and said they were done and he never checked it. :D

[woodtroll]

There is a lot to consider here.
Price of the unit
Availability of accurate base data for the unit.
What data the unit need to show or record.
Cost and simplicity of mapping software for your pc, so you can do  something with the data.
Simplicity of the unit, is it hard to figure out. Can someone who does not want to learn it learn it, even though its "new"?
Product support.

By the way Jim, If it is to gps trees you are harvesting you will soon have a hole in the canopy. right?
Good luck

[jim king]

With our new forestry law the tree has to have a GPS point on the management plan before you cut it and it has to be correct and approved by several government agencies before it can be cut.  Cutting one tree with the branches all intertwined there is not much of a hole if any when you cut it.

I think by studying the internet and talking to a couple of GPS manufacturers it is simply not feasable under a tropical canopy.  Here is some info I pulled off the internet.  Some of it from US Forestry Dept studies.

"Conclusions GPS usage in a tropical forest!

In this study, submeter accuracy was achieved under forest canopy by using Earthmate USB
GPS, but it was not done constantly. For one reason, consumer GPS receivers such as
Earthmate USB GPS are influenced by multipath effects more easily than high-end GPS
receivers that sometimes use multipath rejection technology. In fact, there were multipath
errors seen in the data collected at A5, and positional errors for code-phase DGPS were
unstable especially under forest canopy due to possible effects of multipath. On the other
hand, carrier-phase DGPS with GPS PostPro 2.0 eliminated such negative effects, and
    (submeter accuracy was achieved at all three points after 300 minutes of elapsed time of
measurements.)     As for the distance between the remote and base stations,    (it is recommended
that GPS measurements should be conducted for longer time such as 120 or 300 minutes only)


http://www.ecologyandsociety.org/vol5/iss2/art6/

To measure the positions of trees, we placed the GPS antenna against individual boles in both undisturbed forest and near canopy gaps. If neither GPS nor differential (DGPS) readings were obtained at ground level,   ( trees were ascended using a safety belt and Buckingham tree climbers (essentially, steel gaffs and shanks strapped to the feet).)   Data on the attributes and spatial location of individual trees were then downloaded onto a portable laptop computer and exported into GIS software (ArcView version 3.1, Environmental Systems Research Institute, Redlands, California, USA).
________________________________________
RESULTS
In conditions under which the sky was relatively open and free of obstruction, a minimum of four satellites in optimum position and signals of optimum strength were always located and tracked, allowing consistent collection of high-accuracy data in real time.   (Often, however, ground-based attempts to log differentially corrected data points beneath the forest canopy resulted in total failure.) Although the GPS receiver was, at times, able to locate the required minimum of four satellites, the narrow signal parameters needed for high-quality, differentially corrected positions were rarely obtained.    (  Attempts to plot trees in the forest interior failed because the SNR became so convoluted by heavy foliage that even the minimum requirement of tracking four satellites was rarely achieved)    (Fig. 2A). In trials with the GPS antenna positioned near canopy gaps, the hand-held data logger indicated favorable SNR levels, but these were confounded by conditions of poor satellite positioning or high PDOP (Fig. 2B).    (In an attempt to mitigate these factors, we climbed trees in order to escape as much signal interference as possible. From a height of 25–30 m, the SNR increased and PDOP decreased sufficiently to allow high-accuracy DGPS measurement. This strategy always worked, although the GPS receiver sometimes required 10–15 min to locate satellites with the optimum PDOP required for collecting the highest quality) measurements. In 135 DGPS measurements, we tracked 6.65 ± 0.92 satellites (mean ± 1 SD), and PDOP was 2.48 ± 0.7
FIELD DATA AND ALLOMETRIC EQUATIONS.—We did not attempt to
compare locations of individual trees between field and image data.
     (In a previous study at La Selva, Costa Rica, Read et al. (2003) found
that it was extremely difficult to acquire submeter locations using a
Global Positioning System (GPS) receiver even for large emergent
trees. Location of individual canopies is complicated because of

[woodtroll]

So how will they check you? If you can only have minimal accuracy even with the top end units how can they show your wrong? unless that is the objective.
How good is the base data for your region? Aerial photos? Do you use a mapping program such as ArcGis? If you have photos could you photo reference the points?

I feel for you, being put in a difficult position to meet thoughtless regulations.

[jim king]

The law was made by a group of ecologists that know nothing about the jungle but all are well paid by "Non Profit Organizations".

As for maps , there has never been a study done about what is in the forest or accurate mapping of the Peruvian Amazon.  The coordinates on many properties are off up to 100 miles.

Incredible but true.

[Jeff]

Is there anyway to do some aerial photos and plot gps tree locations from the photos?

[Magicman]

Or have a helicopter hover over you and let him mark a waypoint.  I know chopper time is expensive, but if GPS won't work below the canopy, you might have to get above it.

Remote GPS antenna in or above the canopy.  Let the monkey do it.   

[SwampDonkey]

From what I see in their accuracy (PDOP < 3) they wanted to achieve in those studies you quoted, that's way beyond necessary for your application. You only need a PDOP filter in the neighborhood of 6, which will capture more satellites. In our traverses here everyone sets their units to around 6. Usually in young growth however, we have less than 3 anyway.

I'm taking my GPS up to Jeff's and may have an opportunity to test in his cedar or a maple canopy. As I said, you won't get a continuous lock. I look at my screen for multipath and when it settles down, which is less than a minute in my experience, I take an average. When cruising I always very loosely follow a compass line and when I get close I switch to GPS to go to the point. The compass on the GPS is only a heading your traveling at, no good for traverse compassing.

[jim king]

Is there anyway to do some aerial photos and plot gps tree locations from the photos?

Not that I know of.

Let the monkey do it.

I am sure it would be easier to train the monkey than educate the eco nuts about forestry.

[Clark]

Wow!  Each tree has to have a set of coordinates before it can be cut!  Even if field conditions made that easy to do it would still cut significantly into the bottom line one would think?

The one thing that hasn't been commented on and this probably won't help you Jim, is that some people simply don't get along with electronics.  I had a coworker who was consistently having problems with her electronic data collector.  She finally gave up on it and entered the data at the end of the day, in an office.  I asked her if she ever wore a wristwatch of any sort?  "No, they get messed up and finally quit after wearing them for several months."

I don't the how or the why but there are those people who's body will not allow electronics to operate correctly for very long.  I've only met several but I have no doubt that they would cause a GPS unit to give poor readings no matter how good the conditions.

Clark

[jim king]

Wow!  Each tree has to have a set of coordinates before it can be cut!

Strange but true and the program is backed by the US Forestry Service and the US Government.

[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.