# GPS

Discussion in 'Electronic Basics' started by Rodney Kelp, Sep 5, 2004.

1. ### Rodney KelpGuest

Maybe this is off topic but does anybody know how you convert the gps
position coordinace to distance? For instance If I am currently at
44 degrees 18 minutes 35.3 seconds lattitude and 69 46 52.4 longtitude and
I move to 44 18 35.3 by 69 46 54.0 how many feet have I moved easterly? I
can't seem to find any conversion tables that convert degrees, minutes,
seconds to feet. Am I expecting too much?
My GPS receiver will give me coordinance and speed in miles per hour and
direction but not the distance traveled. What's up with that?
I'm trrying to measure some acerage. I feel like I am going to feel really
stupid when someone tells me.

2. ### JoeGuest

Hi Rodney,

All I know is that mariners use this rule for latitude, 1 minute is a mile
(nautical mile, that is). Longitude is a little more complicated and I think
it depends where on planet Earth you happen to be. Personally, when I use my
gps I keep it in UTM (Universal Transverse Mercator (?)) mode where it gives
coordinates much like a military grid system. Everything is in meters (or
Km). That way, I can see how many meters I have gone and convert (if
necessary) to feet, miles, etc. After awhile, I have gotten comfortable
using meters and Km. If you are moving at an angle, you can see how far
East and North and just use the right triangle rule to figure out your
actual distance change. Much easier.

I never use lat/lon, because, to me, it is just way too confusing. USGS topo
maps list both systems of coordinates, so I use the easier and more
convenient UTM.
From the coordinates you gave above, assuming North latitude and West
longitude, you are somewhere in Maine,USA?

hth,
Joe

3. ### Tim AutonGuest

Yes. If you imagine the lines on a globe you'll note the lines of
longitude get closer together as you apprach the poles - in other
words one degree of longitude is not a constant distance along the
ground. Latitude is easier: one minute is one nautical mile
(approximately).
You need a more expensive GPS

Or you could use this handy page:

http://jan.ucc.nau.edu/~cvm/latlongdist.html

Once you know how far a degree/minute/second is for your location it
should be a good enough approximation for the local area.

Tim

4. ### Rich WebbGuest

For a local approximation, consider 1 minute of latitude to be exactly
1852 meters, which is 1 nautical mile or about 6076 feet. Then 1 minute
of longitude is 1852 meters times the cosine of your latitude. Neither
is strictly true (we live on a lumpy planet) but close enough.

So, you moved 1.6 seconds of arc (0.027 minutes) to the east (assuming

A "flat earth" approximation like this fine as long as you stay in
about the same general area. In the example above, if the starting
points had been 1 nm (1 minute) further north then the change in
distance traveled to the east would be less than an inch.

If you can, record the GPS positions at your boundary spots for some
period of time and pick the center of the spread. This estimate will
probably be your largest error source. Use WAAS or differential if your
equipment supports it.

For larger areas (or more exacting work) you'd need to work in spherical
trig and great circle distances, or go beyond that and work with the
shape of the WGS-84 ellipsoid and the local geoid. But as long as the
"flat earth" approximation errors are less than a tenth or so of your
position estimate errors, you should be just fine in going with the
simpler method.

5. ### KevinRGuest

You just use pythagoras.
Imagine a graph with an X and Y axis and two points on that graph
you'd construct a right angled triangle with one side parralel to the
x axis, another side parralel to the y axis and the hypoteneuse
joining the two points in question.

All you need to do now are convert the results in to feet.

The equator is split in to 360 degrees, each one of these degrees is
split in to 60 minutes and each minute in to 60 seconds
All you need to find out is how many feet is the circumference of the
earth.

Kevin R

6. ### Charles JeanGuest

=============================================================
Hi Rodney,

I agree with Joe. If you've got a UTM setting for lat/long on your
GPS, use it-it's a lot easier. If you don't, but have a topo map of
the area, it's probably what they call a 7 1/2 minute map, which means
it spans 7 1/2 minutes worth of long X 7 1/2 minutes worth of lat. It
will also have line scales at the bottom, in miles, feet, and
meters/km. Since it looks like your units will want to be in feet,
use the feet scale to measure the width(long,A) and height(lat,B) of
the map area. 7.5 minutes = 0.125 degrees, so the conversion factor
from degrees to distance at this point on the earth is A/.125
ft/degree for long and B/.125 ft/degree for lat. Convert your GPS
measured lats and longs to decimal degrees if need be. Then subtract
the lat of the two point and multiply by your lat factor. Do the same
for the long points and multiply by your long factor. This will give
you the number of feet N-S and E-W the two points are from each other.
Triangulation will then give you the straight line distance between
the points.

HTH
Charlie
====================================================================

GRAVITY:

It's not just a good idea-IT'S THE LAW!

7. ### the WizGuest

Easy (but expensive) solution: get the iQue GPS/PDA and the free Cetus software.
Cetus will give the direction, speed and distance (in English or metric units).

VB3/VB6/C/PowerBasic source code: http://www.jecarter.com/programs.html
Freeware for the Palm with NS Basic source code: http://nsb.jecarter.com
Drivers for Pablo graphics tablet and JamCam cameras: http://home.earthlink.net/~mwbt/
Email here: http://www.jecarter.com/contactme.htm

8. ### Rich GriseGuest

A profile view of the earth, exaggerated, would look like the
3-lobed rotor of a Wankel engine, but rounded, of course. I remember
when I was quite young that there was an announcement that scientists
had discovered that the earth bulged, but that the bulge is south of
the equator. And it's kind of elongated at the north pole. One thing
that makes this kinda cool for me is that one time in the USAF I was
BSing with some guy who turned out to be in on the early missile tests
where they discovered the bulge. He said that there was this one series
of flights that kept landing short and to the south of where they had
calculated. (or short and north, whatever). He also said that in one
of their experiments they put a whole rocket in orbit, like, one stage.
At the time, I had thought that that was impossible, like everybody
else. Presumably, it's still up there.

Cheers!
Rich

9. ### JoeGuest

Rodney,

For great circle distance computations it goes like this:

Distance = 69.1 x (180/pi) x arccos | sin(lat1) x sin(lat2) + cos(lat1) x
cos(lat2) x cos(long2 - long1) |