# Determining distance via GPS readings

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Dave King, Feb 15, 2002.

1. ### Dave KingWell-Known Member

Messages:
2,362
Joined:
May 3, 2001
I was working on the "Thing", the Ranging Ballistic Computer and came upon a question/problem.

If distances are determined via GPS fixes, the resultant distances are "base" of the triangle distances and not "hypotenuse". (I hope this is correct). Same as using a topographic map for distance determination, it's all "base" distance.

Laser distances are, for practical purposes, "hypotenuse" distances and need angle correction for "base" distance.

Is there a "Rocket Scientist" here that can perhaps confirm that the GPS distances are "base" distances...

I need to know this as the wind calculation is based on the true and actual flight distance while the "click/elevation adjustment(s)" are based on the gravity or "Base" distance(s).

(I hate it when things are going well and all of a sudden I get wrapped around the axel.)

2. ### Steve ShelpWell-Known Member

Messages:
351
Joined:
May 3, 2001
Dave,
Don't have any experience with any GPS systems but I would assume (yes I know) it would depend on what type of "mode" it was in. If it was in a vector,distance mode it may report back with the true hypotenuse length taking into account the altitude changes also, but if it was in coordinate mode (meaning x,y coordinates only) I would say that wouldn't account for the difference in altitude. In this case I would say that you were recieving just the base distance and not the true distance (hypotenuse) the bullet would travel.

This will be interesting. In HS and College they could have their english courses and Shakespeare, and junk. Me, I love math. Keep us updated on this.

Steve

3. ### Coyote SlayerWell-Known Member

Messages:
62
Joined:
Jul 14, 2001
Dave have you seen the new program from a new company called horus vision they say you can put the program into a palm polit and add wind temp and so on and it will give you an instant amount of MOA this is based on there mill dot scope but i asked if it would work on my nightforce and one of people from the company told me that it will work with any scope made there web page is still under work but you can see the scope and down load the program and see if it works to fit your needs the only thing he said he needed to check was the cold temps we get here:If you give it a look let me know what you think of it:Coyote Slayer

4. ### Dave KingWell-Known Member

Messages:
2,362
Joined:
May 3, 2001
Well, isn't learning great!!

I'm going to try that 1 mile shot again tomorrow and was puttering with the RBC to get an idea of what I'm up to.

I was working on the GPS determination and adding elevation components to get an "auto slope" going. All of a sudden I get this sickening feeling that I'm screwed up. GPS is flat land position!!! There is no slope to take out... for about a quarter of a second I was pretty happy then it occured to me that NOW my WIND CORRECTION is going to be buggered up.

Well, for GPS positions I need to add slope into the distance to get the proper Time Of Flight (TOF) and wind component.

I feel like a need a sword to cut this ever growing problem (Gordian Knot) apart. Maybe I'll just break down and buy a program but then what would I do for mental diversion(s).

[ 02-15-2002: Message edited by: Dave King ]

5. ### Boyd HeatonWell-Known Member

Messages:
1,071
Joined:
May 14, 2001
I love it when you guy's talk about thing's that are WAY over my head.I just start to tingle all over.

6. ### kidcoltoutlawWell-Known Member

Messages:
339
Joined:
Dec 18, 2001
how accurate is the gps,thanks,keith

7. ### titaniumActive Member

Messages:
41
Joined:
Aug 19, 2001
Coyote Slayer,

I have the Horus Palm unit in my hand right now. It works like a charm. The one thing I have noticed about this and one other Palm based ballistics program is that they require some fudging of numbers. What you end up doing is entering velocity, and bullet weight with all the other info, and then keep changing the BC value until it starts to match some known (actually shot) drop values at extended distance. Once you get it to match at say 1000 yards, everything in between is real close. Bruce Krell from www.shootrite.com has one that you input all of your shot data, say 100 yard increments out to 1000 yards and it will calculate every distance in between. He has a 2000 yard model in the works. As far as user interface, the Horus is VERY easy to use, and extremely quick with a result. It also works out to 2500m or something crazy like that. If you have any questions, just ask.

Steve

8. ### Dave KingWell-Known Member

Messages:
2,362
Joined:
May 3, 2001
kidcoltoutlaw

The max distance error for GPS ranging that I have done is about 10 yards.

I don't use the GPS displayed distance, I recalculate the distance in a spreadsheet or on a calculator.