GPS'ing the trees   Paul Jost
  Mar 16, 2004 19:19 PST 

I have access to a Garmin GPS12XL, Garmin etrex Vista, and Magellan SporTrak
Color. The Garmins are similar in reception with the GPS12 only slightly
better than the etrex. The new, enhanced antenna design is on the Garmin
72, 76, and 76S models as well as a similar design in the Magellan Meridian
and SporTrak lines. My brother in Ecuador complains that the Garmin 76
isn't significantly better than his GPS12 under the cloud forest canopy in
steep terrain. My SporTrak is far more sensitive than the GPS12 and etrex
models in my initial testing. Michael Davie has a Magellan Meridian and
complains about problems getting a fix. He was probably using it in hilly
areas, even if on an exposed peak. The problem may be that the newer more
sensitive antenna design is picking up more reflected signals from nearby
slopes that affect the location calculation. I/we need to comparison test
the GPS units in hilly terrain before making recommendations.   

There are SporTrak models in all price ranges competitive with all the
Garmin units. Garmin's user interface (menu structure and keypad) is more
intuitive and user-friendly when on the move. The newer Garmins (72 and
76/76S) are relatively big and bulky, like the Magellan Meridian to
accommodate larger displays for map viewing. The Magellan SporTrak is
smaller and all Magellan Meridian and SporTrak units are about twice as
accurate as the comparable Garmin units. The Magellan SporTrak color,
although very expensive, at the time of my purchase was the only model of
GPS with a 3-axis electronic compass that works when tilted. This is
essential for going to a waypoint without zig-zagging due to GPS tracking

Bottom line--- The Magellan SporTrak series is superior under cover in the
flats but needs to be evaluated in significantly hilly terrain (parallel
ranges of hills exceeding several hundred feet in height). It may prove to
be true that the Garmin GPS12, GPS12XL, and eTrex series might be superior
under cover in hilly terrain if the SporTrak is too sensitive. Testing in
the upcoming months will bear this out...


Paul Jost

-----Original Message-----
From: Gary A. Beluzo 
Sent: Tuesday, March 16, 2004 5:44 AM
Subject: RE: Bullard Woods, MA, GPS'ing the trees

Paul, I'm getting GPS locations indoors on top of my hill in Granby, MA with
a Garmin Etrex Vista ...haven't played with the WAAS mode yet..How much is
the Sportak?


Gary A. Beluzo
Professor of Environmental Science
Holyoke Community College
Re: GPS Equipment    Phil LaBranche
   Nov 17, 2004 06:27 PST 

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   I just got a new GPS last year and it's the etrex Vista. It has many
features and wasn't too bad price wise. Gary Beluzo has the same model. I
saved about $100 or so by getting my unit online instead of at a storefront.
The drawback was that I had to end up waiting for it as it was supposed to
be in stock, but was then backordered. I think the savings was closer to
$150 over what Gary paid for his a EMS, so the wait was endurable for the
savings. My unit was about $150-175, compared to about $300 in the stores.
I used The etrex Vista is easy to use, has an electronic
compass, elevation, save points, mapping, you can download topo maps to your
unit with aftermarket software, is easy to use, and can set your recording
units to various different units (UTM, Lat/Long, ect.) I absolutely love my
GPS and have come across lots of people that have the same make and model.


-------Original Message-------

Date: 11/16/04 23:08:49
Subject: GPS Equipment


What features do you feel are important on a GPS device? What should I be
looking for if I were to buy one? Can anyone recommend a particular model
with a modest price?

Re: GPS Equipment
  Nov 17, 2004 07:48 PST 
I also have the etrex vista. I got it from forestry suppliers. They have five different models from etrex from $119 to $299, depending on what you need. I have had no trouble with it. It does eat batteries though, but it is only two AA's.
Re: GPS Equipment   Don Bertolette
  Nov 17, 2004 16:35 PST 
I wasn't going to weigh in on this one, but would ask if you've thought out your accuracy needs.
For the most part, I am thinking that most of your usage will be in challenging envrionment for most consumer GPSs. If dollars are a consideration, my recommendation would be to look at Garmins, and make sure that you can download your GPS data into a public domain software package like DNR (Dept. of Natural Resources) Garmin (try an online search) that I think comes out of Minnesota (Lee, are you familiar with DNR Garmin?).
For the accuracy I'm guessing you're expecting you need to differentially correct your data...
RE: GPS Equipment   Paul Jost
  Nov 17, 2004 16:54 PST 

I have a Garmin GPS12XL and a Magellan SporTrak Color as well as access to a
Garmin etrex Vista.

As far as reception goes, the two Garmins are similar, but the Vista adds
the functionality of an WAAS correction, a 2-axis electronic compass, and a
barometric altimeter. The compass uses significant battery power but can be
shut off to save power if you don't use it regularly. The digital compass
is useful for going to a known GPS location. Without it, the GPS calculates
and displays a false calculated compass in addition to the arrow that points
you toward your desired destination. However, the calculated false compass
as well as the bearing/course arrows will vary significantly at walking
speeds due to the calculation error in the GPS location as you walk.
Sometimes, walking to a marked point can become frustrating due to the major
zig-zagging that the error creates when walking slowly in dense cover or
over rough terrain. In open terrain, the WAAS correction can reduce GPS
errors from tens of meters down to a few meters by using error correction
data from WAAS satellites over the equator.

The Magellan SporTrak Color adds a 3-axis electronic compass and an improved
quadrifilar helix antenna. A two-axis compass can only display north when
it is held close to horizontal while a three-axis compass will work with the
unit held at any angle from horizontal to nearly vertical. The improved
antenna makes significant improvements in reception under forested canopy
and even indoors. The SporTrak will receive GPS fixes under some conditions
that the GPS12 and etrex units fail under. It should be noted that most
conventional GPS units are designed to have best reception when held
horizontally while quad helix antenna units theoretically work best when
held vertically.

The only known problem with the quad helix antenna design is that it is so
sensitive that it may receive weak reflected signals in addition to the
direct ones. The result is that GPS position fixes may be impossible to
receive at some times in mountainous areas. This has been observed with a
Magellan Meridian that Michael Davies has in his possession. I don't know
if firmware updates ever resolved his problem or if he ever had good success
in mountainous terrain with his unit.

Based upon my experience with GPS units and my understanding of current GPS
technology, I would recommend quad helix GPS units in flat to hilly terrain.
These would include any of the Garmin GPS72 or GPS76 series units as well as
the Magellan SporTrak or Meridian series units. The variations within these
lines depend upon yoru personal usage:
1. Do you desire a compass to align your unit instead of holding a compass
and GPS in the same hand when you walk? If so, get one with an internal
compass. This is an unnecessary, power-consuming luxury for most people,
but can always be turned off to save power.
2. Do you need to download maps into the unit to overlay your waypoints or
routes over? If so, then get one with enough memory to hold the maps that
you want to purchase for your unit. Magellan Meridian units have user
upgradeable memory. Magellan and Garmin GPS units will only allow
downloading of maps provided in their proprietary formats. You can always
download routes without full maps into your GPS. With third-party software,
you can import digitized maps into a PC, plan routes on the PC map, and then
download the route line without the map into your GPS. If you leave your
GPS on to track your hikes, you can download your hikes over the digitized
maps when you get home to have a permanent record of your hike, if desired.
Most GPS units allow uploading and downloading of GPS tracks and waypoints
to and from your PC.

My personal favorite at this time is anything in the Magellan SporTrak
series. I have not used my Garmin GPS12XL or my wife's etrex Vista since
purchasing my Magellan SporTrak Color last year. No, I'm not willing to
give the old one away since it makes a nice backup.

I would hesitate to purchase quad helix antenna units in mountainous terrain
unless we heard positive feedback from users on this list. I don't have
access to areas with parallel ranges of hills much over 400' and they aren't
that really that close to each other, so I can't give constructive feedback
under mountainous conditions.

Any other GPS questions?


Paul Jost
RE: GPS Equipment   Paul Jost
  Nov 17, 2004 16:54 PST 

I assumed that you were looking for a consumer GPS unit for locating trees that others had discovered or for providing site locations to others. I assumed that you were not conducting a study plot to locate individual trees with respect to each other. If you have a relatively clear view of the southern sky down to about 20 degrees above horizontal, then you may, under the best conditions, get accuracy as good as a meter or two with a Magellan WAAS capable GPS if you can lock on to one of the WAAS satellites in addition to the other required GPS satellites of which there are now 31 or more. The Garmin WAAS corrected error is about twice that of the Magellan units. Under non-WAAS conditions, errors of 10-15 meters are more typical and can be much worse in heavy cover or rough terrain. Do not use the EPE or estimated position error calculations of the GPS as true errors. This is best done by using the unit near a surveyed location and comparing the known reading to the displayed reading. Public surveyed locations are found across the country and can be searched for online. 

WAAS satellites provide real-time differential correction data that provides better results than that provided by the US Coast Guard for marine purposes. Better accuracy is probably only possible with expensive survey grade instruments.

Paul Jost
RE: GPS Equipment
  Nov 18, 2004 04:15 PST 
Here in Se Pa, with clear skies, I have received readings down to seven feet with my vista. It depends on time of day and obstruction. The WAAS is very helpful. We had some surveyors to our property to plot some wetlands for the Army corp of engineers, they were able to get down to less than a quarter of an inch with their equipment. I was impressed to say the least.