GPS

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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

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:

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

--
Guns Don?t Kill People, Rappers Do.

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.

--
Rich Webb   Norfolk, VA

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

============================================================= 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!

I

and

really

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) |

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:

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