[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

RE: GPS, navigation (long)

>> UTM is easiest to measure from topo maps:  Units are decimal
>>and scale is constant...

I worded that poorly.  The distance between the UTM analog of "longitudes"
is constant.  Since longitudes converge at the poles, the distance between
them varies.  Therefore, you can't use a simple map-overlay gadget to plot
longitude.  UTM pretends that the earth is flat over small areas.  True-
north lines on the earth's surface cannot be parallel, thus the divergence
of grid north from true north.  The divergence is minimized by dividing the
UTM grid into zones which are re-oriented to north.  A similar phenomenon
occurs in states which use the township/range/section system of real
estate:  They must correct the north direction occasionally, which produces
those odd ~200-foot offsets in n/s roads at township boundaries.

We landlubbers may observe the curvature of the earth with a pair of
binoculars and a body of water 1-2 miles wide, e.g. a straight stretch of a
large river.  Stand at the water's edge and observe the far shore.  Then
squat or assume prone position, and watch the target disappear behind a
"hill" of water.

BR>>"One other interesting complaint, was echoed by Jeff Schaffer, one of
the authors of the PCT trail guides. The USGS topos are not accurate, so
when you get a good GPS fix, you cant place yourself on the map. The topos
are based on aerial photos, and some features may be off by as much as 1/2
mile or more. This presented problems to the PCT cartographers when they
made the maps for the guidebooks, because their ground surveys turned up
the errors. They chose, for example, to show the PCT crossing a pass at the
col on the map, even though the col was in the wrong place. The typical
user will compare the map with what he sees, and head for the col, instead
of using surveying tools. They did this to make the guidebook maps useable
for the hiker -- and they did a great job."

Hadn't heard about that!  All topos are made from aerial photos but the
photos are calibrated by ground targets referenced to USGS benchmarks.
Blunders occur, of course!  Topos are rarely updated but sectionals are
republished twice per year and user input about corrections is solicited.

>> I'm used to lat/lon; I use aeronautical sectional charts for
roadmaps, especially "out west."...

Do you mean for just road driving, or walking to?

The scale is a bit large for walking (1:500,000; ten statute miles = 1.25
inch.  Contour interval is 500 feet (sometimes with 250' intermediate
contours)).  They are usable for long-distance hiking, and are far better
than the Rand-McNally Road Atlas which an acquaintance used when he paddled
a canoe from Bloomfield, Indiana to Del Rio, Texas.

Sectionals' dimensions vary; most are 8 degrees wide, 4 deg. tall.  It
looks like 4 of them would cover the AT (no, it's not marked):  Atlanta,
Cincinnatti, New York, Montreal.  Their north/south halves are printed on
opposite sides of the paper, so you would need two sets (and a tall wall)
to construct a wall map.

>> GPS web site:  http://www.tech.net/technotes/gps
>> The FAQ on Garmin products includes software reviews.

>the site is strickly Garmon? You didn't say if you have used the
other GPS 'puter prgs I mentioned?

It includes much general GPS info.  I haven't used any GPS computer

>>>7.What are the best distances apart to use for waypoints, for
hiking? (5, 10, 30, etc?)

>> ...Perhaps set significant destinations, or just turn the unit on
occasionally to check progress and record present position each time.  On a
well-marked north-south trail, latitude alone is sufficient for a fix.

>Not clear what you mean, marked on the map or stored in the GPS, or both?
Like I said in my post, not going on a well marked tail, starting on CDT in
NM, were there isn't much marked trail, and a lot of cross country.

Lat/lon of features marked on sectional charts can be estimated to the
nearest 0.1 minute (about 600 feet).  You could try measuring and storing
prominent landmarks like mountain peaks, radio towers, intersections of
trail with roads and rivers, etc.

>Would it be simpler to use a aero plotter and maps, then program the legs
into GPS?  Is this one way you do? Its true course, as I'm not going to
have compinstate for wind drift!

That's how I do it for road trips.  The GPS receiver automatically computes
and displays the directions and distances between series of waypoints.
Since you can't "go direct" on land, course deviation is far less important
than bearing and distance from present position to the next waypoint.

In any case, devise some exercises and get very familiar with GPS in the
field before a "real" trip!  BTW, most handheld GPS have single-channel
receivers and non-preamplified antennas:  They may lose satellite signals
in "dead spots" that occur under dense forest canopy and in narrow valleys.

>> GPS altitude is nearly worthless...

>I don't need to worry about crashing into a mtn, ie, adjusting my
altimeter for sea level barometric pressure,  just worry about the strain
of getting up and over!

An altimeter becomes a land-navigation instrument when using topo maps or
sectional charts:  When the road crosses a contour line, you have a fix.  I
love the aircraft altimeter in my vehicle.  Indication can be estimated to
five feet but it's too big for backpacking.  A friend who thru-hiked the AT
last year had THREE Avocet electronic wrist-altimeters die on him and
concludes that they aren't serviceable.

BTW, if you take an aircraft altimeter into the field for surveying, etc.,
protect it with a foam-rubber beer-can insulator and a neck lanyard.  A 3-
foot fall onto a hard surface will trash it beyond economical repair.

>> A rule of thumb for estimating walking speed:...

The "ranger rosary" sliding-beads-on-parachute-cord gadget available at
army-surplus stores is a sort of one-row abacus / low-tech pedometer useful
for counting hundreds of meters (or hundreds of paces if you prefer non-
metric).  The package includes instructions.  Lacking that, you can count
to 35 on your fingers in base-6.

Garmin model 38 has a resettable "trip odometer" function, model 45 does
not.  Battery consumption prohibits continuous GPS use on a long hike
unless you want to hassle with solar cells.

--  Frank     reid@indiana.edu