Georeferencing Map and Image (Vector and Raster) - 01

When we doing a project we will find data and maps are in several formats and media. In ArcGis we can use some data format type there for first we have take these data in to soft copy. In some cases, one of your data sources may be in the form of a paper map, a scanned version of a paper map, or some other digital image which does not contain spatial reference information. Scanning a paper map produces a raster data set that can then be used in a GIS project, once it has been georeferenced.

What is Georeferencing ?

When we scan a map it is just a image or raster image. For this image or raster we can assigning a coordinating system or process of assigning geographic information to an image in known as Georeferencing. Knowing where an image is located in the world allows information about features contained in that image to be determined.  This information includes location, size and distance. 

Typically used for

            - Satellite images

- Aerial photographs

            - Scanned CAD drawings

Geographic data conversion

Two main approaches for converting information on hard copy maps to digital data. It is callas Scanning or Digitizing

You will need at least two layers:

1)      Layer that needs to be georeferenced

2)      Layer that is already georeferenced (Basemap) 

Georeferenced Imagery

The Earth has been divided into a grid with lines of Longitude and Latitude.  When an aerial photograph is aligned to this grid, it becomes "georeferenced."  Georeferenced image allows the following details to be obtain from Georeferenced map:

-  Distances can be measured

-  Areas can be measured

-  Directions can be determined

-  Exact position of any pixel can be determined

How to do georeferencing an image in Arc GIS

                Following are the steps to be adopted to do a georeferencing image. This Process of georeferencing image included

•          Add image to map (unknown location)

•          Add reference data to map (known location)

•          Find control points

•          Check error (RMSE)

•          Save georeferencing

Add image to map (unknown location)

First we have to add Layer that needs to be georeferenced to the Arc Map

Add reference data to map (known location)

                Find out the reference point or known locations

Find control points

Control points are the known location such as;

-          Street intersections, corners

-          Buildings

-          Other landmarks that do not move

These control points used to register dataset to known location in the map. Before we georeference the image we have to find out good control points.

Criteria for control points:

-          Spread evenly around study area

-          Minimum number of points (at least 4)

-          Use highest accuracy dataset possible

-          Use locations identifiable on both datasets (known & unknown)

Final result:

-          Your image is saved with new georeferencing

Step by step Georeferencing with ArcMap 10

- Open ArcMap 

- Select Customize – Toolbars – Georeferencing

- In the menu bar, select File – Add Data - Click on the file you want to Georeferencing

- Get the X and Y coordinates to be assaying (This methord we used when we know the X and Y. If we don’t know the X and Y locations, still we can do georeferencing. I will cover it in Georeferencing Map and Image (Vector and Raster) – 02 section)

- Get the X and Y coordinates to be assaying (This methord we used when we know the X and Y. If we don’t know the X and Y locations, still we can do georeferencing. I will cover it in Georeferencing Map and Image (Vector and Raster) – 02 section)

-          Zoom the location you want to add 

 - Using the add control points tool bar add the points. (Minimum 4 Points)

- Click the point and press the right hand button and enter the X & Y coordinates according the X and Y coordinate (See above given Map diagram).

Root Mean Square error (RMSE).

A measure of the difference between locations that are known and locations that have been digitized.RMS error is derived by squaring the differences between known and unknown points, adding those together, dividing that by the number of test points, and then taking the square root of that result. This should be less than 1 

Then Rectify the image.

Image rectification is a transformation process used to project two-or-more images onto a common image plane. It corrects image distortion by transforming the image into a standard coordinate system.

Makes the bounding

You can omit the unwanted area of your map to make your area of interest. To do this you will required Clip tool. Clip tool makes bounding rectangle to be clipped. To make sure that the proper alignment is used.

- Geoprocessing – ArcToobox- Data Management tool – Raster - Raster Processing - Clip

- Select the input file image

- Enter data to X Min,Y Min, X Max, Y Max

- Select the output file with extension (.img)

Thank You