There's an app for Android that does this. It's called Measure Map. It is accurate to whatever accuracy your phone GPS has. Easy to use, just mark your location, go to the target, measure back to your original location. Think they have it for iPhone also....
Before lasers became reasonably available--or at all--during the eighties I used a Ranging 1000. It's a co-incidence or "stereoscopic" rangefinder wherein one brings the two images together. It's in like new condition--I'd ship to you for a hundred bucks if you want it.
Go on Ebay and the WW2 militaria section. Once there, type in rangefinder.
They aren'talways on there but do show up occaisionaly.
At this point in time, id suggest a Wild, due to the age factor on most others, although ive seen post war Barr & Strouds also.
Also, one never knows for sure how long they will last. We still have a WW1 unit in use by a camp member and its still in fine shape.
Expect to pay three to five hundred for one in good condition, not to say you wont encounter ones at higher prices.
Check with Deitsch optic in California, as they have had them from time to time also.
They had lots of new never issued Wilds for as I recall $300 about 10 years ago.