Georeferencing in gis means a process of importing digital image and adding geographic coordinate in it so that the GIS software can locate in its appropriate real world location. The digital image may be an air photo, topographical map, a scanned geology map, etc. In other words, it is a process of relating the digital image to the internal coordinate system of GIS software. The coordinate systems are made up of some specific datum.
Georeferencing definition- According to USGS, "Georeferencing means that the internal coordinate system of a digital map or aerial photo can be related to a ground system of geographic coordinates."
Why do we need georeferencing process in mapping?
Georeferencing in gis is a preliminary step of any mapping and analyzing process.
Map making software such as ArcGIS, QGIS, etc. cannot proceed in analyzing
process and others without completing georeferencing step. It is mandatory to
georeference of an image it may aerial photograph, a topographical map or a
picture of drone. We have different type of coordinate system to take one of
them for georeferencing. Coordinate system can mainly divide into two broad
categories which are Geographic Coordinate System and Projected Coordinate
System. Both coordinate systems have different purposes in different
aspects. It is depend upon what topic we choose and what type of project we
have. Most common coordinate system WGS 1984 is used all over world in
different projects.
Steps of georeferencing in arcgis
Two different processes are available in most
the of map making software which are by adding coordinate value (X, Y) in known
crossing point and taking reference data (reference data should be
georeferenced). These two process or methods are mostly used for
georeferencing. Below the methods are shown step by step
The Steps of georeferencing in arcgis are shown in below-
By adding coordinate vale (X, Y) in known crossing point
Step-1
Go to catalog and connect your folder where your
raster data is present and add the non-reference raster data.
After adding the raster image (non-reference data) you need to specify a coordinate system. Here I am going to put WGS 1984 geographic coordinate system. For specifying coordinate system go to Layer→ Properties→ Coordinate System→ Geographic Coordinate Systems→ WGS 1984
Step-2
After adding raster data you have; then add
georeferencing tool by going to Customize Manu→ Toolbars→
Georeferencing.
Step-3
Zoom in the map by using Zoom in tools and
then click on add control points tool and add the control point on the crossing
known point as like the picture shown in below-
Note:
To add control point you have to right-click on the
crossing point and to input DMS you have to simply left-click. And make sure your
hemisphere in which your map falls. Here, my map falls in Northeast hemisphere.
In this way, you have to
enter minimum four control points like below the picture-
 |
|
|
|
|
Step-4
To evaluate the transformation of the digital image, click the View Link Table.
In this step, you have to examine the residual error for each inputted link and the root mean square (RMS) error. If you are satisfied with the registration, you can stop inputting control points.
Step-5
Now you can update or rectify your georeferencing image. For that click on georeferencing drop-down Manu and click either update georeferencing or Rectify tool.
Note-If you update your georeferencing image or transformation information then it will save with raster and its auxiliary files. One thing is that the update information could not save permanently but whenever the program closes all the data will be erased.
So if you need the
transformation information permanently then you have to rectify the georeferencing
image or the transformation information. What will be happened if you rectify,
it will create a new file with the georeferencing information.