GPS and/or topo maps for ranging

[Len Backus]

GPS and/or topo maps for ranging
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Anyone out there with experience or practical knowledge on the use of the civilian GPS unit and/or topo maps for determining range.
I've been toying with the idea of recording the coordinates of various known 'hot' spots onto my library of topo maps. I figure the GPS can get me a 'fix' to within 10 yards and that if I know the sites the animals present in I can use the previously recorded position of the animal and my current position to get a line-of-sight distance. I don't need to worry about the functioning of the laser unit and the distance for fixes on the GPS are not limited to 1000 yards or so.

Any thoughts?

posted April 13, 2001 08:00 AM

Warren Jensen
Member

From: Arco, ID US
Registered: April 12, 2001
Posts: 16
GPS for ranging.
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With the GPS units that I have used, and my experience is not extensive by any means, it is possible to get different location readings for the same place on different days. It is usually not a great difference and is probably a function of which and how many satellites you are receiving at that time.
I would be real interested in how this works out for you Dave. I have had some ideas along these lines myself, but have not had the opportunity to pursue it.

[email protected] http://www.lostriverballistic.com

posted April 13, 2001 04:00 PM

Ken Howell
Member

From: Stevensville, Montana
Registered: April 12, 2001
Posts: 5
When my son was using the GPS in surveying Forest Service lands here several years ago, I asked his boss — surveyor for the Bitterroot National Forest — how accurate GPS surveying was. I was especially interested in its distance-measuring accuracy, having been "brought up" on the 66-foot steel chain and disappointed in my one use of stadia in the Arctic.
His answer was that GPS was VERY accurate (I forget the error level) in measuring the distance from Point A to Point B but not nearly so accurate in establishing where either point was, relative to the rest of the world.

I can not guess whether this tells you anything useful. I'm not a bit familiar with either GPS as used with the instruments for land-surveying or GPS as used with the hand-held doodads available to ordinary citizens.

I'd check this out with some up-to-date surveyor

posted April 13, 2001 06:29 PM

Len Backus
Administrator

From: Oshkosh, WI
Registered: April 10, 2001
Posts: 21
GPS
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Since last year on 5/1/00 "selective availablity" has been turned off. This was a Department of Defense built-in random error.
Warren, now the expected accuracy is supposed to be in the range of 10 to 20 yards. Have you tried your repeated tests since that date?

I sold my GPS about 4 years ago when I figured out in the field how variable the reading for a specific location was. Now that SA is turned off I may purchase one again.

Len Backus

posted April 13, 2001 06:37 PM

Warren Jensen
Member

From: Arco, ID US
Registered: April 12, 2001
Posts: 16
GPS
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No, I haven't tried the repeatability test since then. I will, and good point Len.
[email protected] http://www.lostriverballistic.com

posted April 16, 2001 07:53 AM

 

[Lee Owens]

On the Message Board at www.varminthunters.com a guy who goes by "Mike SSS" posted a formula for converting minutes and seconds of Lattitude and Longitude into yards of distance. This was over a year ago, while I was in the US Army as an M1A1 tanker. I bought a Garmin and tested it against the M1's Laser Range Finder. The formula works like a charm. I have since lost the formula, but you guys have given me an excuse to ask for it again.

 

[Larry in WI]

While in South Dakota recently on a PD shoot I was fooling around with my etrex summit, lo & behold it really works. I would walk the town and enter waypoints then I could just ask for the waypoint from my bench(start point) and the gps would give me a bearing and distance, worked very well. Accuracy shown on the screen was a 23 foot error correction.The unit only cost $250 bucks and is well worth it.

 

[ChadRack]

I think GPS and topographic maps go together great for ranging. I use it to plot locations of deer stands on our lease and also to make sure that they are far enough apart.

The units with the position averaging feature work best. UTM is the easiest way to tranfer coordinates back and forth between the unit and the map, but in can be done in DMS if you have an older map. Most local map stores carry the see-thru scales, but you can make one also. I take the difference in the "Northing" and "Easting" and use standard math on the angle to figure the distance. Then repeat if there is a large difference in elevation. I've haven't tested this since they turned the SA off, but accuracy should be in the 10' range now. The more readings you take, the more accurate your answer will be.

 

[Dave King]

Larry in Wi

What units(yards, feet, meters) did the GPS display for distance and what level of magnitude (10s or 100s of feet or yards)? I have a Garmin 12 and it's display units are too course to be of much value for ranging at any appreciable distance.


Thanks

 

[ChadRack]

On your 12XL, switch from DMS format to UTM. The coordinates are now in meters. When you you go to the position page, you'll have a "Northing", that's for vertical, and an "Easting" for horizontal. Take an average position at point 1 and an average at point 2. Now take the difference between the vertical readings for 1 and 2, and the horizontal readings for 1 and 2. These give you the 'rise' and the 'run' for the triangle and you can now figure the 'diagonal' which will be the distance, in meters, between 1 and 2.

If there is much difference in elevation between the two points, then you'll have to take this into account also. Just figure another triangle. Use the topo to find difference in elevation as GPS has some trouble with altitude.

Your handheld unit is much more accurate than the .01 mile display. Some of the newer units switch to feet once you get close to your desired waypoint. This is nice, but not as accurate as averaging both positions. Hope this helps.

 

[cronhelm]

I used a Garmin GPS II to measure a 1900 yd shot a few weeks ago. This was a borrowed unit so I was not familiar with all the bells and whistles but it was reasonably easy to use.

The unit showed the range in 10m increments. I do not know if it is possible to increase the sensitivity or if this is as good as it gets.

I think that 10yds is probably good enough. Conventional wisdom is that one only needs to know the range to within 25yds of the actual distance to get onto the target.

At these sorts of extreme ranges there is a lot more error in the ballistics table than there is in a GPS ranged distance. I did some reading and the G1 drag model that most ballistic computers use is not at all good at estimating drop when velocities are close to or below the speed of sound.

In my case I found that the actual drop was about 23 MOA greater than the calculated drop.

If you want to read about my 1900 yd adventure and see a few pictures go to:
http://www.nucleus.com/~cronhelm/index.html

FWIW This shot was performed with a run of the mill .243 Winchester and a 1:8 barrel.

Peter Cronhelm

 

[Dave King]

Chadrack

Thanks for the 12XL info, I'll give it a try.

Cronhelm

I read your article, good reading. Welcome to Long Range Hunting, I believe you'll enjoy the place.

 

[Dave King]

Chadrack

System works like a champ. Spent most of my life as a sailor and always used Lat. & Long. This UTM is new to me but the distance calculations are very simple, now I need to add a calculator to my hunting list.

Thanks

 

[Dave King]

Still checking this UTM distance calculation again't the laser. Last night I did a check at 766 lasered yards and got 2 GPS fixes for a GPS distance of 761 yards (used 1.095 as the meters to yards conversion). I stopped at the Maptools WebSite yesterday and downloaded a set of UTM grids for ranging and printed them onto transparent film.

Are there any civilian GPS units out that give the distance in between two points in meters or yard directly? (My GARMIN 12 gives distance in 10ths of a mile.)

 

[anyrange]

Just got an eTrex Mariner, next high power rifle silhouette match I will enter way points for target line and each firing line out to 1000 yards. Your idea should be within 10 yards if satellites are good.
I enter a point in the pistiol range parking lot and then walk around and navigated back to to way point. Was off by about 20 feet. Did it three times and had nice 5 foot group 20 foot from entered way point.

 

[ChadRack]

Dave, that's great accuracy! You'll really like the transparent UTM grids. I've been wanting to get out and test accuracy since the SA is off, but just haven't had time yet.

Any day the new I-Finders will be out. Never thought I'd have any use for a background map, but with the new software that's coming out, they're like having a topo inside your unit.

I think there is also a 'position correction factor' on your unit that will align your GPS to your map. This is a must when using a topo.

I took a surveying class in college and the major factor in better accuracy was the amount of readings their unit took for average. There was no difference in the magnitude of change in position between it and my old 8-channel handheld.