Editing Scale (map) (section)

== Large-scale maps with curvature neglected==
The region over which the earth can be regarded as flat depends on the accuracy of the [[surveying|survey]] measurements. If measured only to the nearest metre, then [[curvature of the earth]] is undetectable over a meridian distance of about {{Convert|100|km}} and over an east-west line of about 80&nbsp;km (at a [[latitude]] of 45 degrees). If surveyed to the nearest {{Convert|1|mm|inch}}, then curvature is undetectable over a [[meridian arc|meridian]] distance of about 10&nbsp;km and over an east-west line of about 8&nbsp;km.<ref name=merc>{{citation|first=Peter
|last=Osborne
|title=The Mercator Projections
|year=2013
|doi=10.5281/zenodo.35392
|postscript=. (Supplements: [https://zenodo.org/record/35561 Maxima files] and  [https://zenodo.org/record/35562 Latex code and figures])
}}</ref> Thus a plan of [[New York City]] accurate to one metre or a building site plan accurate to one millimetre would both satisfy the above conditions for the neglect of curvature. They can be treated by plane surveying and mapped by scale drawings in which any two points at the same distance on the drawing are at the same distance on the ground. True ground distances are calculated by measuring the distance on the map and then multiplying by the [[Multiplicative inverse|inverse]] of the scale fraction or, equivalently, simply using dividers to transfer the separation between the points on the map to a [[bar scale]] on the map.