Georegistration is the process of registering and transforming a raw map or raw satellite image in order to assign it real-world coordinates. When we scan a map, it is not referenced with the earth and when we open such scanned map in GIS software whatever coordinates it displays, those are coordinates of computer screen not that of real-world. Hence we need to assign proper coordinates to it and transform it into a georegistered (or georeferenced) map having real-world projection system.
A raw map can be georegistered in two ways-
1. If the hard copy map contains coordinate information in it (mostly written as latitude/longitude on the border of map at regular intervals), then this map can be georeferenced using its own coordinates. After scanning such map we import it in GIS software and assign same coordinates on the same places as marked on it.
2. If we don’t have a map as discussed above then we have to georeference it using another map which is already georeferenced. In such cases we have to identify same features in both maps and need to take control points on both them one by one. After taking sufficient points we georeference the raw map.
Control points are those features which are identifiable on raw map/image and on corresponding georeferenced map/image (as well as on ground).
Avoiding Errors in Georegistration
After scanning a hard copy map and importing it into GIS software, we need to georegister it. Generally it is considered as the first step to begin a GIS task (unless you already have georeferenced maps or images). Georegistration is important process as positional (or geographical) accuracy of features to be shown in subsequent maps depend on it. Most of the times errors come when control points are less, inaccurate and not homogenously distributed throughout the scanned map (or satellite image).
One should be very careful while marking control points if there is any confusion in identifying features on raw map and georeferenced image then one should always confirm the right location. If it remains doubtful then its better not to mark any control point. Even a single wrongly marked control point could create significant positional error.
Apart from selecting accurate control points, selection of right polynomial equation for transformation (or resampling) the raw map/image is also important. One should always experiment- which order of polynomial can give desired results. Generally it is said that higher the polynomial order higher accurate will be georegistration but sometimes using higher order may skew or stretch your map too much and the map got deformed (it mostly happens when there is less number of control points on one portion of map while more on others). So one have to apply one’s own judgment that which order is best. Suppose you have taken 12 control points for georegistration, now you have options for using polynomial order 1, 2 or 3. It is suggested that one should use all the three options and observe the result to decide the suitable one.
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