BajaNomad - Finding a point on a map

Finding a point on a map

Bruce R Leech - 4-11-2005 at 07:23 AM

dose any one Know how I can find the GPS coordinates for a point if I have the distance to the point and the bearing from where I'm standing. for example if I'm standing on a spot with my GPS in hand and I need to no the coordinates for a point 2000 feet away at 280 deg heading mag.?

Easiest way, Bruce

neilmac - 4-11-2005 at 10:38 AM

is to hire a surveyor and have him figure it for you... BTW, I work really cheap in Baja... can start right away.

If you have a geodetic traverse program, it's pretty easy... if not.....

1. switch your GPS from lat/lon (degrees) to UTM coords (meters North and East); and convert your distance from feet to meters (divide by 3.2808333)... 2000' = 609.6 m.

2. determine the bearing of your line of sight... bearing is the angle between your line and a due north line... with a quadrant identifier.... azimuth 280 = North 80 d. West.

0 to 90 is NorthEast
90 to 180 is SouthEast
180 to 270 is SouthWest
270 to 360 is NorthWest

3. calc the 'latitude and departure' of your line... latitude is the north/south component of the line, departure is the east/west component...

uh, hope that's what you wanted to know... if ou know a surveyor, ask him to show you how they do it.

Neil
latitude = cosine of the bearing times the distance;
cos(80) x 610 = 106m

departure = sine of the bearing times the distance;
sine(80) x 610 = 601m

since you're on a NorthWest bearing, ADD the latitude to your estarting North coordinate, and SUBTRACT the departure from your starting East coordinate... (if it was a NorthEast bearing, you'd add them both; SE is subtravt N, add E, SW is subtract both).... and you now have your new coordinates... key them into your GPS and switch it back to lat/lon for degrees, if you want.

if you're a real masochist (or surveyor), measure the bearings and distances around a closed figure; that is, come back to your starting point on the last leg..... then calc all the coords using the above method... and see if the last coord is the same as the first.... (or if you can live with the difference.)

Oh, yeah....

neilmac - 4-11-2005 at 10:42 AM

If you are using a compass for bearing, subtract the declination from your magnetic azi to get a 'true' azimuth

Neil

bajajudy - 4-11-2005 at 10:51 AM

Got that, Bruce?

Bob and Susan - 4-11-2005 at 11:38 AM

This is the EASIEST way....

WALK the 2000 feet and take another reading on your GPS unit :yes:

Bruce R Leech - 4-11-2005 at 11:44 AM

Quote:

Originally posted by Bob and Susan
This is the EASIEST way....

WALK the 2000 feet and take another reading on your GPS unit :yes:

that is cheating this needs to be solved from the known point. pretend that you are nailed down. :lol:

wornout - 4-11-2005 at 03:03 PM

Quote:

Originally posted by Bruce R Leech

Quote:

Originally posted by Bob and Susan
This is the EASIEST way....

WALK the 2000 feet and take another reading on your GPS unit :yes:

that is cheating this needs to be solved from the known point. pretend that you are nailed down. :lol:

I agree with Bob and Susan but if you have to pretend you are nailed down, get a friend to walk the 2000 feet and take a reading. :lol:

[Edited on 4-11-2005 by wornout]

BillB - 4-11-2005 at 07:51 PM

I think most GPS units have this feature built in to them as well. I don't have my GPS unit (Garmin) handy at the moment, but when you are in the mode where you can see the map, you can "pan" the map using the cursor, and if you have the settings right... it will display the distance and heading the cursor is from your current position.

I use this a lot to keep myself entertained while flying across the country. If I see a small town out my window at a distance... I try and guess how far away it is, then move the cursor until it is on top of the town, and see how far away it is and on what heading. Yes, I spend way to much time in airplanes... lol

I know what you are thinking... is using a GPS in an airplane legal? Truth is each airline has their own policy on it, but I have never been bothered. I practice the don't ask, don't tell policy. Actually on two occasions I have had off duty pilots sitting next to me and they were both rather amazed at what I was able to show them.

Bruce R Leech - 4-11-2005 at 07:59 PM

sounds like you have a pretty nice unit. mine doesn't have mapping software. it is the smallest Garmin.

bajalou - 4-11-2005 at 09:28 PM

Bruce
If you had a program such as OziExplorer it's a snap. Just put your wayoint in from your GPS, click on the "show distance and bearing display" and move your mouse the distance and bearing you want. Then make a waypoint there in the program, load it to your GPS and you're all set.

:biggrin:

BillB - 4-11-2005 at 09:51 PM

Forgot one thing Bruce... when your cursor shows that you are far enough in the right direction, you can enter that new location as a waypoint, and can then use it as if you had actually walked the 2000' or had a friend walk it for you!

BillB - 4-11-2005 at 09:55 PM

Bruce... not sure what Garmin you have, but when I said go to the map page, it is the same page that may just show you the breadcrumb trail of where you have been, and the relative location of other nearby waypoints.

I am now using a Garmin Vista and an older Garmin IIIplus. Both have this feature.

David K - 4-11-2005 at 10:01 PM

Bruce must have the base model (yellow) eTrex, which was what I started with. It doesn't have the pan feature... I didn't discover that feature until I got an eTrex Legend (Blue).

[Edited on 4-12-2005 by David K]