How to Georeferencing in ArcGIS

Introduction

                                          There is a great deal of geographic data available in formats that can not be immediately integrated with other GIS data. In order to use these types of data in GIS it is necessary to align it with existing Geographically reference data this process is also called Georeferencing. The process of Georeferencing relies on the coordination of points on the scanned image [data to be georeferenced] with points on a geographically referenced data [data to which the image will be georeferenced]. by "linking" point on the image with those same locations in the geographically referenced data you will create a polynomial transformation that converts the location of the entire image to the correct geographic location. call the linked points on each data layer control points. 

The selection of control points is important:

1. They should be easy to confirm as representing the same geographic location (landmark, political boundary, etc...).
2. They should be spread across the image to be registered one suggestion is to select a control point near each of the corners of the image and few throughout the interior will often work well.
3. Good overlap between the two often dataset is also important.
4. Make sure you are clicking as close possible to the same geographic location, zooming in can help in this process.                                                     

What is Georeference?

                             Georeferencing is the internal coordinate system of a map (or) aerial image can be related to a geographic coordinate system. The relevant coordinate transforms are typically stored within the image file there are many possible mechanisms for implementing georeferencing,...

How to Georeference in ArcGIS

                                Georeference is the process of transforming a scanned map (or) aerial photograph so it appears in place in gis. by linking, connecting (associating) features on the scanned image with real world x, y coordinates. The software can progressively warp the image so it fits to other spatial datasets. This tutorial will explain how to georeference a raster image in arcgis so it can then be used as an overlay or for digitizing purposes. in this example _ a historic Toronto map will be georeferenced using a dataset.

Process,- 

(Georeferencing a Raster to a Vector)

Open ArcGIS(ArcMap):- 

STEP-1 in ArcMap add the layer residing in Map coordinates and add the raster dataset you want to Georeference.

Adding  the data with the map coordinating system first is a good practice so you do not need to set the data frame coordinating system. Note-(study area Change the coordinating system to suit you will be selected)

STEP-2 To display the Georeferencing toolbar click the customize menu and click toolbars > Georeferncing. 

STEP-3 in the table of contents, right-click a target layer and click zoom to layer.

it may be helpful to set your Extent used by Full extent Command, within the Data Frame properties, to your study area so the Zoom to Full Extent tool will automatically zoom to the full extent of your study area.

STEP-4 From the Georeferencing toolbar, click the layer drop-down arrow and choose the raster layer you want to Georeference. 

STEP-5 Click the Georeferencing drop-down menu and click Fit to Display. This displays the raster dataset in the same area as the target layers. You can also use the Shift and Rotate tools to move the raster dataset as needed. To see all the dataset, adjust their order in the table of contents.

SETP-6 Click the Add Control points tool  to add control points.

SETP-7 To add a link click a know location on the raster dataset, and click a known location on the vector layers in map the reference data.

You can also your link in the magnification window (or) the viewer window. if you are using polygons as your referenced layer, you can open the effects toolbar to adjust the transparency as you add your links.

SETP-8 Add enough link for the type of transformation you will apply. 

You need a minimum of 1 link for zero-order polynomial, 6 link for a second-order polynomial, and 10 link for a third-order polynomial (or) spline transformation.

SETP-9 Click the view link table button  to evaluate the transformation.

You can examine the residual error for each link and the root mean square RMS error. If you are satisfied with the registration, you can stop entering link.

SETP-10 Delete any unwanted link from the link table.

SETP-11 Click the Georeferencing drop-down menu and click either Update Georeferencing or Rectify. Updating the georeferencing will save the transformation information with the raster and its auxiliary files. Rectifying will create a new file with the georeferencing information.

     You don’t need the control points anymore.

     Keep the magnifier window open since you may want to use it when digitizing  the layer.

Note - Georeference Complete and Save as Referencing Map End...

ArcMap Georeference Practice Video...