v. 2008, Build 5.1

Raster Image Calibration and Georeferencing

Mapwel provides two approaches to establish relation between raster image and map coordinate system. In both cases, result is a map with Lat/Lon grid parallel to the screen edges and coordinates assigned to left, right, top and bottom edge of the map. Selection of proper approach depends on the type of the raster image:

Approach 1  (recommended)  is a direct map calibration. This calibration transforms raster image and vector data drawn on top of the image to the Lat/Lon grid parallel to the screen edges. This method uses an affine linear transformation and will work with maps which are rotated or skewed. The Lat/Lon grid of the image must be linear (straight lines) to allow linear transformation to the grid parallel to screen edges.  Even images with a non-linear Lat/Lon grid can be calibrated with this tool, if the grid non-linearity is caused by projection (like UTM, for example). In such a case, map usually contains linear Easting/Northing grid and non-linear Lat/Lon grid. Calibrate map with use of the Easting/Northing grid and Mapwel recalculates images so that the Lat/Lon grid becomes linear.

Approach 2 involves separate transformation of raster image into rectangular grid, and consequent map georeferencing. If image (typically distorted scan or map with unknown projection) is curved in a way that direct calibration cannot be used, it can be edited with tools for rotation, move, cropping, non-linear straightening and morphing. The goal is to achieve Lat/Lon grid parallel to the screen edges and optionaly to make raster image match the tracks and waypoints imported from GPS. Once the image is calibrated, map can be georeferenced with use of 2 points.