| Laser
tests |
Robert
Leverett |
| Jan
18, 2006 11:11 PST |
ENTS,
It is easy to always trust
your laser rangefinder. But constant
checks and new calibrations are a part of our craft. The table
below
contains the results of a test conducted in a light rain using
two
lasers, an old Bushnell 800 meter/yard model, which I have
always
regarded as my most reliable LR, and my Nikon Pro Staff 440.
Interestingly, the Nikon shoots half a yard shorter to a bright
target -
at least in rain.
When shooting a trees on
which I want to achieve high accuracy, I
use both lasers, shooting multiple times in different lighting,
and
ultimately settling on a measurement that I think I can defend.
If I'm
not looking for extreme accuracy, I just use one laser.
I'll repeat the test below
in sunlight and report the results for
the stationary objects in the list.
Bob
In rain/drizzle
Yds
Yds
Target
Bushnell
Nikon
Wall of bldg next 75
75.5
to sign - dark
Sign on wall-bright
75 75
Wall of bldg adjacent 75 76
to a sign - dark
Sign on wall-bright
75 75.5
Base of pin oak-dark 79 79
Red car door-fairly bright 36 36.5
Base metal light pole - dark 54 55
Bright yellow pipe #1 55 55.5
Bright yellow pipe #2
42 43
Bright yellow truck 71 71.5
Metallic gray car door
76 76.5
Yellow sign
103
104
Beige Building side 157
158
White shade in window 154
154
Red taillight
32
32
Immediately adjacent
32 32.5
to taillight
Tip of white pine 102
103
Avg
76.06
76.62
Robert T. Leverett
Cofounder, Eastern Native Tree Society
|
| Re:
Laser tests |
foresto-@npgcable.com |
| Jan
18, 2006 12:57 PST |
Bob-
Of course, in any such comparison, I will pipe in that you
should get a more
accurate distance measurement using a reflector than you would
surfaces with
less accurate reflectance values...what were the actual
(measured by steel
tape, for example) of the red reflector?
-Don
|
| RE:
Laser tests |
Robert
Leverett |
| Jan
19, 2006 06:45 PST |
Don,
For shooting to the trunk, reflectors are
often practical. Of course,
they don't apply to crown shots. We're usually shooting multiple
tops of
the crowns of trees to find the highest top. So we need to know
how our
instruments perform under less than ideal conditions.
Though less tahn ideal is a given for crowns,
less than ideal often
applies to trunks as well. Sometimes we're on the opposite sides
of
streams from the trees or the trees are in dense rhododendron or
laurel
thickets. On very important trees, I do often place a more
reflective
object on the trunk of a tree and shoot it as opposed to the
darker
trunk.
With respect to the lasers, my objective was
to test them under
typical field conditions where the targets have different
reflectivities, where atmospheric conditions vary, the surface
of the
targets are different (rough, smooth, etc.), and where distances
to the
target vary greatly. So, I upped the number of tests to 32 with
distances from 21 to 152 yards. Conditions varied from rain to
cloudy to
partly cloudy to sunny. Targets were of all types and textures.
Under
this wide range of conditions, the average difference over all
distances
between the lasers stands at 0.95 feet and the difference is
partly due
to the fact that the Nikon reads to the half yard and the
Bushnell to
the whole yard. Shifting around showed me that if I got to click
over
point in each case, the 0.95 feet went down to an average of
about 0.5
feet. So over the wide range of fiend conditions, the two lasers
are
pretty close. Results are closest at the shorter distances. At
disatnces
of 40 to 80 yards, the average distance without adjusting for
click-over
is 1.0 feet. Getting to click over reduces this to about 0.5
feet.
Tests of lasers calibrated on changes of 1
yard at a time to the same
target can give an incomplete picture. To understand on'e
instrument,
one needs to use targets of different shapes and reflectivities
and in
different atmospheric conditions. The full range of values
eventually
show themselves.
Bob
|
| RE:
Laser tests comments continue |
Robert
Leverett |
| Jan
19, 2006 09:43 PST |
Don,
More thoughts on lasers and laser tests. Colby
Rucker was a strong
proponent of eliminating potential error whereever one could.
So, he ws
a strong proponent of controlling the error of eye-level to the
ground
measurements by using a pole with a flag on it. Howard Stoner
also make
good use of the pole-flag apparatus. It is the better way to go.
Whether one employs a pole, a reflector, or a
reflector on a pole, it
makes sense to reduce potential error on the lower traingle
where that
can be done. However, I would emphasize that may measurers
really don't
know their equipment well enough over the range of conditions
they
encounter. There is variability among instruments due to:
1. Shape of object,
2. Reflectivity of object,
3. Distance of object,
4. Atmospheric condition between laser
and object.
5. Interference by surrounding objects.
Of the 7 laser rangefinder I've owned, one
Bushnell shoots long by a
yard (the 500 yarder) on the first measurement and then often
settles
down to the right yardage. An older 400 yarder perpetually
shoots short
by a yard. The other two Bushnell's are highly accurate. Or I
should say
one is. The other had to be laid to rest. It went to laser
heaven.
The Optilogic instrument sucks, but is useful
for very short
distances provided it has a wide and long unobstructed path to
the
target. The first Nikon seemed okay, but is dead. It drowned. I
don't
think it went to laser heaven, so I'll say no more about it. The
second
Nikon is the one I've been reporting on. It is a good instrument
that
picks up targets with more clutter around them than the Bushnell
except
when the rain mode is invoked to shoot through brush. That mode
doesn't
always work, but it is as effective as the Nikon, if not more
so, when
it does work.
The latest generation of lasers now beg to be
tested. Wish I had one
of every make and model.
Bob
|
| RE:
Laser tests |
foresto-@npgcable.com |
| Jan
19, 2006 20:05 PST |
Bob-
I agree with everything you said. I am certain that your
comparisons as
provided were exhaustive (even exhausting?).
The 'fields' I would put across the top of the chart, to get at
what you
haven't said, are (IN CAPS)
[natural target surface = nts; reflector = R; taped measure =
tm):
nikon (nts) ~ bushnell (nts) ~ NIKON (R) ~ BUSHNELL (R) ~ actual
(tm)
When running traverse in deep forest with Criterion 500, we
would often turn
on audio signal...the Criterion would only pick up the reflector
signal in
this mode, and only when it had enough for an accurate reading.
Many times,
it would 'educate us' where the foresight (guy at the next
point, with
reflector) was...it would find it when we couldn't find it by
naked eye. It
would seem to me to be a good feature for your lasers when in
rhododendron
patches!
-Don
|
| RE:
Laser tests-back to Don |
Robert
Leverett |
| Jan
20, 2006 05:48 PST |
Don,
No question about it, I'd love to have the features you
describe.
Anti-rhodo and laurel capabilities would be sooper dooper.
Howeevr, it
sounds like the Criterion 500 is pretty expensive. Do you know
its cost?
Is/was it made by LTI? I think Van Pelt had a Criterion 400 and
it does
a few things that his Impulse Laser doesn't do. At least that's
what I
seem to remember him saying. I think he took diameter
measurements with
his Criterion 400.
BTW, my tests of the Bushnell versus Nikon
don't identify the more
accurate of the two, just how one compares to the other. Over
the next
few weeks, I'm going to bring out the old tape measure and do
more
comparisons.
One final point is that I could mimic my
roving tests using your
ground level plan in all ways except possibly one - background
lighting.
Shooting at the top of a crown with sky beyond presents a range
of
lighting conditions that would be hard to duplicate shooting at
ground
level. Dark target, bright sky, Bright target, dark sky
(hmmm-tree on
fire?), bright target, bright sky. Dark target, dark sky. Any
ideas?
There are times that my Bushnell shoots long by a yard when I go
slightly above the crown point. Why would I do this? Because
dropping
the laser much beneath the crown point to get a fimer target
also
introduces the problem of hitting forward thrusting twigs. One
develops
a sixth sense about hitting forward thrusting targets, but then
the
process of measuring becomes more art than science.
Bob
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