From OpenStreetMap Wiki
Jump to: navigation, search

Enhanced Mapping with DigiCam

Calculation of Distances

As I have no real access to the railway-stations rail-yard. It is very difficult to estimate locations of signals and switches. Therefore I took some geometrical help. As on some Digital cameras the focal length is well known and then sensor-size (the size of the projected image in the camera( is also well known following calculations are possible:

Calculating Distances with known Original-Size

I took the following Input-Values:

  • Original Size of Object (Railway-gauge = 1435mm) om
  • Horizontal Sensor-Size (22,2mm) = sm
  • Real Focal Length of a image (eg. 108mm) (see below) = f
  • Image-Size in pixel (3456px) = ip
  • Image-Size of the object in Pixel (90px) = op

With these information a simple math-solution is possible to create in every spreadsheet-application:

  • Object-Size on the sensor osm = sm * op / ip (22,2mm*92px/3456px)
  • R = osm / om
  • distance d in mm = f * R

Now anyone can say that the size of the sensor chip and this pixel-size of the object is very difficult to get. I agree with that, but getting a Pixel-Size of 88 to 92 Pixels the Distance of the example above varies between 223 and 233 meters.

Some Point-And-Shoot Cameras with an intergrated Photographic lens have a focal length given that equals the 35mm-format. For the real distance the real focal length is needed:

  • real Focal Length = 35mm-format focal length * sensor-width in mm / 36mm

Calculating Distances from size-distance-change


  • Object-Size in pixel from Distance 1 (s1) = l1
  • Object-Size in pixel from Distance 2 (s2) = l2
  • let there be s1 > s2 and so l1 < l2
  • Changed distance in m (as precise as possible) s1-s2 = ds

With these information a simple math-solution is possible to create in every spreadsheet-application:

  • size-difference in percent p = s2/s1 - 1
  • s1 = ds / p
  • s2 = ds+s1

Note: This only works with 2 pictures taken with the same focal length and the object well centered in each image.

Enhanced Mapping using a compass

Position, Distance, Bearing

Knowing an Object and its real Distance from the current location (e.g. with Photographic enhancement) it is posible to take a bearing with the compass and create the position in JOSM by using the measurement-tool in the status-bar. It is important to know the local magnetic declination to bear according to true north and not magnetic south.

cross-bearing a position

When there is no posibility to get a GPS-Position at an object like an electricity pylon in the middle of an acre it is possible to estimate the position through cross-bearing

Therefore three steps have to be done:

  • mark current location A and note true bearing from A to Object
  • mark current location B and note true bearing from B to Object
  • Use JOSM and its measurement-tool and create a Way from A with the noted bearing A-Object and from B with the noted bearing B-Object.

The crossing of the two ways mark the Position of the Object. Optimal positions A and B are when the difference between A-Object and B-Object is aproximately 90°. For better Estimation more than 2 bearing-positions could be made.