GPS question [RE-wrenches]

[Matt Lafferty]

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Brother Jay asks:

> Say was wondering if anyone is using a GPS instead of a compass these  
days?
> And on the same idea, for altitude vs barometer?

All,

Yes and Yes.

I have been using GPS units for Solar orientation verification since 1998.
In that time I have owned 4 different models priced from $99 to $300.  I
have used several others and tried to help other Solar folks figure out how
to get the right info out of theirs.  (I'll explain some of the dangers
involved in this in a moment)  Only one really cuts the mustard in a
reasonable price range.  The others just cut the cheese. I swear by, instead
of at, one and only one. 

The winning and ONLY GPS that I recommend for Solar folks is the Garmin
E-Trex Summit.  These run about $225.  I do NOT recommend the E-Trex.  Don't
be fooled by the pretty yellow (or blue) case of the E-Trex!  Get the gray
on black model. Don't be lured by the "$50 cheaper" price tag, either. Get
the E-Trex Summit.  No joke.  (I'm sitting in the office of the PV
Contractor who physically installs more Mega Watts per year than anybody
else in Northern California right now.  There are three "no longer used"
E-Trex's sitting here because they did not take my advice on this three
years ago. If anybody else doesn't want to take my advice on this and would
like a deal on them, I'm sure that can be arranged.  You don't want to hear
about or re-live the headaches and frustration associated with this, trust
me.)

The key ingredient is a reliable "Compass" feature that reads accurately at
a standstill.  All units will give you a "heading" while you are moving at
some set speed... Generally 4-6 MPH as a default.  Others have a "Compass"
but it requires moving at a minimum speed as well.  BEWARE!  This is not
always clearly evident in the specs and brochures!  It can be very dangerous
to get an accurate reading while walking down the slope of an 8:12 roof at 6
MPH!  Especially the ones that only have 12 foot spans. It is also very
important to realize that it is humanly impossible to walk in a perfectly
straight line at that speed, even on level ground.  Your overall line might
be "straight", but the GPS refresh rate doesn't match your stride.  Try it.
You don't realize how much your body moves side-to-side when you walk until
you've done all this in side-by-side tests.  I hate to say this, but my
observation is that ladies' strides end up having heading readings that are
mind-bogglingly all over the place compared to a man's, even when they are
walking right down a painted line.  My best guess is that it is all in the
hips and the relationship between the stride and refresh rate of the GPS.
(I'm really not joking here.  If any of you have lady surveyors out in the
field and rely on "movement-dependent" GPS's, you should really be paying
attention!)  The point is that you want a Compass feature that does not
require movement to give an accurate reading.

The Summit has a sub-feature to the Compasss called "Sight and Go".  This is
the one you want for solar layouts.  Basically, you hold the thing up to
your eye, line up two points on the unit with your object or the direction
you want to sight, and click "Enter".  This "freezes" or "holds" the
orientation and display so you can look at it and get the exact bearing.
This is the ticket for many things.  Should be your default use for all
sightings, actually.  When laying out reference stakes or landmarks for a
ground-mount survey you can't beat it.  

An important feature is to be able to select "magnetic" or "true" as the
base North reference.  Summit has it.  (Matt T: Don't assume that your
Meridian is giving you a "True" reading!  Go into the settings and make
sure!  Hey, 16 degrees off when you're over on the East and West sides of
things makes a hell of a difference!) 

Another thing I do is record the GPS location of every site right on the
Survey form.  I've done that since 1998.  One of these days I'll be able to
have a nail polisher go back through thousands of old surveys and input all
the info. We input all the current ones into the database now.  One of these
days I'll overlay a map with them and have "popup" system data when you
click the icon, including "rejected for shade, orientation, etc."  No sense
going to a bad sight with a new owner five years down the road!  Unless of
course one of my Lotto tickets pays off before then.  I've done some
preliminary development on that.  If I wasn't such a bumbling "programmer"
I'd be further along on it... If only I'd gone to college... If only I'd
gone to college... If only I'd gone to college... Nah. Wouldn't trade the
scrapes and scars... 

As far as the altimeter goes, calibrate it and calibrate it often if you
need gnats-ass.  Calibrated, they are plus or minus 3-5 meters, like the
Altitech 2. Which, at first glance, adds up to a lot of head one way or the
other on a Hydro system.  Or those cases when you leave your Pathfinder home
and have to climb a tree to figure out how tall it is to see how far the
shadow will creep on December 21.  The reality is that, for most of our
purposes, we really are doing "elevation difference measurements".  If you
are doing altitude/elevation measurements between two points on a site,
calibrate it just before taking your first elevation.  Take your second
elevation reading before turning it off. Generally, the "mean-sea-level"
altitude is far less important or completely irrelevant than the elevation
difference between two points. For elevation-difference measurements on a
site, the difference is what matters, not the actual "altitude".  For this
purpose, they are actually very precise.  For those altitude dependent
aspects, they are not generally so touchy that the tolerances of the unit
will mess you up.  This goes for all the hand-helds that I've used. 

I don't "mark" my sites in the GPS memory.  Don't tell the IRS, but I do
mark my personal sites.  Even an ex-girlfriend's house is still in there, I
think.  I never really used it for guiding me from point to point until
February of this year while boating over on the West Coast of Vancouver
Island.  Funny how all them shorelines look the same when it's dusk and
you're trying to get back to a certain point by "gut reckoning" in a 10 foot
Zodiac from out in open water.  My Summit never lied, even when all my
senses thought they knew better.  It was fun watching the speed, too. Maxed
out at 7-8 MPH when hauling tools and supplies back and forth.  About 18 MPH
with just me in the boat.  16-17 with two people on board.  (For what it's
worth, I was up there installing a PV system and doing some fix-it, so you
can tell the IRS about that one.  In fact, I'll tell them all about it on my
tax return so you don't have to.  Funny how the expense receipts added up to
so much more than the income.  I'm gonna have to quit working for friends, I
guess.  Does anybody know if they make you declare the fresh oysters, clams,
and mussels we ate as income?  Yummmm!)

I don't get into all the map-loading stuff, so I can't comment on that
aspect.  (Roy B: Guess they frown on just pulling off the side of the road
and taking a leak out there in NY, huh?  ;-) )

A couple of words of advice when using a GPS for bearing.  Don't take your
readings when standing next to the wall of a house or under an eave. There's
a little "signal bounce" effect that skews the reading.  GPS's like a clear
view of the sky. Carry a good compass and spare batteries for backup.  Know
how to use your compass.  Check your GPS against your compass regularly.
Especially when there's a nagging voice in the back of your head saying,
"But I know South is over there!"

Two cents that I hope prove helpful for many.

Pray for Sun, Ya'll!

Matt Lafferty
mlafferty at universalenergies.com

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