How do i determine the viewing elevation of a scaled map?

Question

Lets say a map is scaled at 1:20,000. What would the elevation/altitude of the center of the map be at this scale? In other words, if a straight road were exactly 20,000 inches long how high would my eye need to be before I observe the road to be 1 inch long? It seems like there would be a standard conversion but I cannot find it anywhere. thanks.

Answer 1

20,000 inches i would presume

Answer 2

Go up about 1670 feet above the terrain.  

Where are you holding the ruler to make the measurement?  Take a piece of paper and draw a 6-inch line on it.  Put the paper on the floor.  Now pick up the ruler and bring it closer to your eye as you read the ruler.  That 6-inch line appears to get shorter the closer the ruler gets to your eye.  You don't get a certain distance above a map to make it look like the real thing.

Scales for various projects and maps need to be different according to the requirement.  The more detail, the better the accuracy and expense.  The scale for laying out an airport does not need to be as accurate as the scale for laying out the terminal.  The alignment of the runway can be off a few inches, but not acceptable for the terminal.

Try looking at a location using http://zillow.com.