Both Leupold and Bushnell offer rangefinders that will calculate true, horizontal distance from an angular distance. In general, the more expensive models will range farther. Don't be too impressed with hype that they will range objects out to 800 or 1000 yards. They may range a white barn at that distance, but won't range a deer sized critter anywhere near that far. I have a Bushnell rangefinder. Don't recall the exact model name, but it ran around $250. Once, out of curiousity, I aimed it at some cattle near where I was hunting. Couldn't get a reading off it. Found a white steer in the herd and tried it. Distance was around 375 yards. Worked on a white steer at that distance, but not a black one. It's supposedly good for a deer sized critter out to 200 yards. I take it with me hunting, sometimes, but have never used it for that purpose. For the ranges I generally shoot deer at (rifle hunting), I really don't need it.
Always enjoyed watching The Best of the West on TV. The former host used a rangefinder unlike any I'd seen in the catalogs. Always figured it was some kind of military model. They would range elk out past 800 yards with it on TV. Stumbled across it by accident on-line recently. It's an XL-20 (or was it XR?) rangefinder. It will range a target out to 2000 yards or meters, give or take 1 meter. Costs around $1,700 and weighs over 2 lbs.
