About Contour & Grid
Contour & Grid is a free set of map and GPS calculators for hikers and orienteers, geocachers, geography and GIS students, field researchers and anyone who has to move a position between a phone, a paper map and a written report. It covers the arithmetic that sits between a coordinate and a decision: how to write a position in another format, how far apart two points are, which way to head, what a measured map distance means on the ground, how large a boundary encloses and how steep the ground is between two contour lines.
The Calculators
- Coordinate Converter — decimal degrees, degrees-minutes-seconds, decimal minutes and UTM grid references on WGS84, including the Norway and Svalbard zone exceptions.
- GPS Distance Calculator — the shortest distance on the WGS84 ellipsoid with Vincenty's method, compared with the spherical haversine result.
- Bearing & Midpoint Calculator — initial and final great-circle bearings, the back bearing, the midpoint and the compass point.
- Map Scale Calculator — map distance to ground distance and back, and the scale from two known distances.
- Polygon Area Calculator — area and perimeter from a list of corners, in square metres, hectares, acres and square miles.
- Map Slope Calculator — average grade, angle and slope ratio from the contour interval, contours crossed and map distance.
Why We Built It
Most map mistakes are not about hard mathematics. They are a dropped minus sign, easting and northing swapped, a percent grade read as degrees, a datum ignored or a bearing used without its declination. Each of our calculators shows the working and the format of every result, so that you can see where a number came from and catch these errors before they matter on the hill or in a report.
Methods and Sources
All positions use the WGS84 ellipsoid (semi-major axis 6,378,137 m, flattening 1/298.257223563), the datum used by GPS. UTM conversion uses the transverse Mercator series published by the US Geological Survey (Snyder, Map Projections: A Working Manual), accurate to well under a metre within a zone. Ellipsoidal distances use Vincenty's inverse formula; spherical distances and bearings use the haversine and standard great-circle formulas with the IUGG mean radius of 6,371,008.8 m. Polygon areas use the spherical excess formula with the authalic radius of 6,371,007.2 m. Map scale and slope calculations are exact arithmetic on the values you enter.
Who Uses It
Hikers writing a route card, search-and-rescue volunteers practising grid references, geocachers converting puzzle coordinates, students checking homework on map projections, ecologists recording plot corners, drone pilots planning a survey area and landowners estimating a field before ordering seed or fencing.
Keeping It Accurate
We test the converters against published reference points and round-trip every UTM conversion back to latitude and longitude. Distances are checked against known values, such as one degree of longitude at the equator, 111.32 km on the ellipsoid. If you find a result that disagrees with a professional GIS package by more than the stated precision, tell us the inputs and we will investigate.
Independence and Funding
Contour & Grid is independent and not affiliated with the USGS, Ordnance Survey, any mapping agency, GPS manufacturer or app. It is free and supported by advertising; advertisers have no influence over the content.
Privacy
The coordinates you enter stay in your browser and are never sent to us, which matters when they describe your home, a private site or a sensitive location. See the Privacy Policy.
Contact
Corrections and suggestions: logan.luo@adsnova.cn.