Also, when you say "reliable", I think you mean accurate. Do you have a particular use in mind? Whether you're talking about something you carry with you or a tool for reading a map, all technologies have limits. Then there are the deep philosophical questions: when you step up and over a boulder, should you count the rise + run + drop, or just the horizontal distance?
Planning a ten-mile hike and getting the distance right to within half a mile might be good enough for you, in which case lots of methods will work. If you need to get much more precise than that, you might need a whole lot more planning and equipment.