# Digital Geography

26. September 2012

# ArcGIS tutorial 1: how to georeference a map

ArcGIS is a geographic-information-system (GIS) developed by ESRI. Being widely used and the sheer amount of functions and add-ons are maybe the important causes for being a big-player at universities as well.

Therefore we will present you some basic tutorials for learning an enjoying ArcGIS:

Tutorial 1: how to georeference a scanned map in ArcGIS?

### objective

To connect a scanned map with other geographical data the map needs to be refrenced. This meens, that every pixel in the scanned image gets a new dimension: besides its location in the image (e.g. 326th pixel from left and 124th pixel from the top) it gets a real-world coordinate as well (e.g. 45°32′ and 16”N 120°37’11”W ).
The objective of this tutorial is to show you how to translate these pixel-coordinates in real-world coordinates, aka. georeferenciation

### what do you need

ArcGIS 10, 9.3, 9.1, etc.
a scanned map availbale as *.tif or *.jpg (other flavours can be used as well, but should be ratser)(we’ve used a topographic map 1:25.000 (4521) distributed by the “Hessisches Landesvermessungsamt” (1983)

### step 1: define a projection

In the first step we need to define a projectionfor the map: so the same term is also the function in ArcGIS: Define Projection (Abb.1).

Fig.1 Location of „Define Projection“ in the ArcGIS Toolbox

In the tool Define Projection we choose the scanned map as Input Dataset (Fig.2 red 1.) and the coordiante system that is defined in the description of the map (Fig.2 red 2.).

Fig. 2 Input of parameters

### step 2: preparation of the georeferenciation

Now we need to adjust the visibility of the Georeferencing Toolbar. Look at Fig. 3 an follow the four steps.

First open the Editor (red 1.). There we choose Toolbar options ->Customize (red 2.) and a list with available toolbars will be opened (rot 3.). Now search for the Georeferencing and check it. The Toolbar shows up in your working window (red 4.).

### step 3: georeferencing

Zoom to the upper left edge of the map and search for a crossing of a longitude and latitude. Now create a point on this cross using the function Add Control Points (Fig.4 red 1.). After marking the point use a right-click (red 2.) and choose the option Input X and Y: a window will open (red 3.) where you can record your pairs of input and output coordinates. Record the values that belongs to the latitude and longitude as Y- and X-value. Do this for the other three edges of the map as well! if you don’t want to struggle with auto-projection use always opposing pairs (lower left, upper right, upper left, lower right).