Georeferencing and Digitizing in ArcGIS

Digitizing & Scanning in ArcGIS Prepared By: O.A. OPALEYE

20th October, 2009.

Digitizing

Digitizing is the process of drawing or tracing a paper map on a digitizing tablets or onscreen; thus converting features on the paper map or aerial photographs into digital vector format. Digitizing can be done in two ways: 1) 2)

Tablet Digitizing On-screen Digitizing

Tablet Digitizing

You can use a digitizing tablet connected to the computer in which the map document is fixed to trace the features you want to digitize. The puck: a handheld device on the digitizing tablets or table is used for digitizing the graphic objects. The x, y coordinates of these features are recorded and stored as spatial data. On-screen Digitizing

Feature on the paper map can be traced using the mouse What do you need?

1) 2) 3) 4)

Graphic Scanner connected to a computer system Source Map or aerial photograph Installed Scanning and Image editing Software ArcGIS, Arcview or any other GIS software

What to do? a) Scanning b) Establish Control Points c) Start Digitizing (A)

Procedure for Scanning your Paper Map

1) 2) 3) 4) 5) 6)

Switch on the scanner Place the paper on the glass plate, image down (depending on the kind of scanner) Start the scanning software: Photoshop, Corel-draw etc Procedure – File-Import-Acquire File-Import-Ac quire (the name of the scanner) Select one of the scanning options: colour, grey scale or line art. Select the resolution; which is the number of pixels per inch, the greater the resolution, the better the picture. For on-screen digitizing, a resolution of 200dpi (dot per inch) is mostly sufficient.

7)

8) 9) 10) 11)

(B)

Make a preview scan (a rough scan will be made in order to preview the image to determine the area to be scanned by dragging a box over the image After scanning, save image. You can save the image in different formats, like: .tiff, .jpeg, .bmp etc. In Photoshop – Go-to- File-Save as- type in your desired name and change the format type and click ok  In Core-draw – Go-to- File- Export- Type your desired name and change the format type and click ok  Geo-referencing a Raster Dataset

Raster data is commonly obtained by scanning map or collecting aerial photographs and satellite images. Scanned map don’t normally contain spatial reference information. Thus, to use some raster dataset in conjunction with your other spatial data, you will need to align or geo-reference to a map coordinates systems. Steps for Geo-referencing a Raster dataset in ArcGIS Adding the raster data

1) 2) 3)

Click the add data button on the standard toolbar to add the raster data Navigate to where your data is and add Click set to display

Adding Control Points

4)

5) 6) 7) 8) 9) 10) 11) 12)

Identify at least four known points (x, y) on the map or data. Control points are locations that can be accurately identified on the raster dataset and in the real world coordinate. Load your Geo-referencing Geo-referenci ng tools from view-toolbars-geo-referencing view-toolbars -geo-referencing Pick the add control point tool and add your selected points Click the mouse over the known point on the raster layer, then right click and select “enter x, y” (coordinates). Type in the x, y coordinates of the point. After you have added at least four (4) points, you can click the link table to examine your root mean square (rms) error. You can delete control points with greatest residual errors and repeat adding control points Click “geo-referencing” “geo-referencin g” and “update geo-referencing” to save the transformation information with the raster data Your image is geo-referenced and you can start your on-screen digitizing.

Creating a Personal Geo-database Using ArcCatalog

1. 2. 3. 4. 5.

Launch the ArcCatalog Open a your Working Folder Right click in the folder and point to New Click Personal Geo-database ArcCatalog creates a new personal geo-database in the location you selected and sets its name to edit mode. 6. Type a new name for this personal geo-database How to create feature dataset

1. 2. 3. 4. 5. 6.

Right click in the geo-database Point to New Click feature dataset Give name and click next Specify Coordinates system and click next Finish

How to create Feature Class 1. Right click in the Feature dataset folder 2. Point to New 3. Click feature class 4. Give name and class 5. Specify feature class type (Point, Line or Polygon) 6. Finish.

On-screen Digitizing Creating Point Feature

1) 2) 3) 4) 5) 6) 7)

Load the point shape file created in ArcCatalog Click the editor tool and click start editing Pick create new feature from the “task” drop-down box. Pick the point feature dataset you want to digitize from the “target” drop-down box. Click the sketch tool from the editor tool palette Click on the map to create the point Point will be created on the map.

Creating Line Features

1 2. 2) 3)

Load the line shape file created in ArcCatalog Click the current “task” drop-down box and click the create new feature Click the “target” drop-down box and click the line layer Click the “editor tool” palette drop-down box and click the sketch tool

4) 5) 6)

Click on the map to digitize the feature vertices When finished, right click anywhere on the map and click finish sketch The line feature is created on the map.

Creating Polygon Features

1) 2) 2) 3) 4) 5) 6) 7)

Load the polygon shape file created in ArcCatalog Click the current “task” drop-down box and click the create new feature Click the “target” drop-down box and click the polygon layer Click the “editor tool” palette drop-down box and click the sketch tool Click on the map to digitize the feature vertices When finished, right click anywhere on the map and click finish sketch The polygon feature is created on the map. Pick auto-complete polygon tool to append adjacent polygons.

Reference: 1) ArcGIS 9.2 Desktop Help 2) ITC Lecture Note: Demonstration on Preparation for Digitizing