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

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  1. Rodney Kelp

    Rodney Kelp Guest

    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. Joe

    Joe Guest

    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?

  3. Tim Auton

    Tim Auton Guest

    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
    You need a more expensive GPS :)

    Or you could use this handy page:

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

  4. Rich Webb

    Rich Webb Guest

    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
    your positions above are N and W) or about 116 feet.

    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. KevinR

    KevinR Guest

    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

    Kevin R
  6. Charles Jean

    Charles Jean Guest

    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.



    It's not just a good idea-IT'S THE LAW!
  7. the Wiz

    the Wiz Guest

    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).

    More about me:
    VB3/VB6/C/PowerBasic source code:
    Freeware for the Palm with NS Basic source code:
    Drivers for Pablo graphics tablet and JamCam cameras:
    Email here:
  8. Rich Grise

    Rich Grise Guest

    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. :)

  9. Joe

    Joe Guest


    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) |
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