GEOREFERENCING
Geo-referencing refers to aligning geographic data to a known coordinate system so it can be viewed, queried, and analysed with other geographic data. Geo-referencing may involve shifting, rotating, scaling, skewing, and in some cases warping, rubber sheeting the data. Geo-referencing can be done using ArcMap. Before we perform geo-referencing, we will obtain an image from Google Earth and thereafter geo-reference the image in ArcMap.
SETTING THE COORDINATE SYSTEM AND THE UNIT OF MEASUREMENT IN GOOGLE EARTH
To set the coordinate system and the unit of measurement in Google Earth we have to go to Tools => Options as it has been shown by figure 1 below.
Figure 1: Setting the coordinate system and unit of measurements
After we have clicked the "Options" option, we will be provided with the screen shown by figure 2 below. To complete the settings, we will click on OK.
Figure 2: Setting the coordinate system and unit of measurements
Viewing grids on Google Earth
To view grids on Google Earth, we will go to View => Grid as it has been shown by figure 3 below.Figure 3: Viewing grids
After the grids have been enabled, we will be able to see the zone we are working on in the coordinated system that is being used. In this case the UTM coordinate system. We will select the Universal Transverse Mercator (UTM) coordinate system. We will set the units of measurement as Meters, Kilometers.
Adding Place Marks
Before we add the place marks for georeferencing, we have to tilt and compass the area that we are going to place the place marks. To tilt and compass, we will go to View => Reset => Tilt and Compass as it has been shown in figure 4 below.
Figure 4: Tilt and Compass
To add place marks of an area in google earth, we will go to Places => Add => Place Mark as it has been shown in figure 5 below.
Figure 5: Adding place marks
We will place the place holders on google earth and thereafter record the coordinates of the place holders. Figure 6 shows the place holders placed on google earth.
Figure 6: Place holders on google earth
After the place holders have been created. We will save the google maps screen as an image. To save it, we will go to Save => Save image as it is shown in figure 7 below.
Figure 7: Saving the image with place holders
After the image has been saved. We will open it in ArcMap. To load the image in ArcMap, we will set up the folder connections of so as to allow the image to be added as data. When we load the data into ArcMap, we will get the message that the data being loaded is lacking a special reference as it is shown in figure 8 below. This is because the data is loaded as a normal image without a defined coordinate system. We will click the OK button an proceed.
Figure 8: Unknown spatial reference
After the image has been loaded on ArcMap, the next step is to open the georeferencing toolbar. In order for us to open this toolbar, we will go to Customize => Toolbars => Georeferencing as shown in figure 9 below
Figure 9: Accessing the georeferencing toolbar
After we have enabled the georeferencing toolbar, the toolbar will be visible, and by using it, we
will manage to georeferenced the image. Figure 10 shows the georeferencing toolbar of ArcMap 10.x and its respective controls.
Figure 10: Georeferencing toolbar
To start georeferencing the image, we will click the add control points button. After we have
clicked the button, the pointer of the mouse will change its shape to a plus sign. We will therefore
have to click a point on the image and thereafter specify the coordinates of the point. The coordinates that we will use in the performing of geo-referencing are the ones that we obtained from the control points that we placed on Google Earth. To place a control point on an area of the image, we will have to zoom to the control point of the image, thereafter go to the georeferencing toolbar and click the add control points button, thereafter right click and go to input X and Y data as it is shown in figure 11 below.
Figure 11: Adding control points
After we have clicked the Input X and Y … button, we will be provided with an input box which will allow us to insert the X and Y coordinates of the point as shown in figure 12 below. This process will be repeated until when all control points have been finished.
Figure 12: Entering coordinates
ArcMap as well allows us to view the error that occurs as a result of georeferencing. This error can be viewed through the link table in the georeferencing toolbar. Figure 13 shows the link table used in this project.
Figure 13: Link Table
After we have finished inserting the coordinates of all control points, we will go to the georeferencing toolbar and rectify the Geoprocessing process as it is shown by figure 14, below.
Figure 14: Rectify
After we have clicked the rectify option, we will be provided with a screen which will allow us to save the georeferenced image. Figure 15 shows the screen.
Figure 15: Saving the georeferenced image
After we have finished saving the georeferenced image. We will remove the image from ArcMap and thereafter open ArcCatalog in order to specify the coordinate system used in the georeferencing process.
SPECIFYING THE COORDINATE SYSTEM
To specify the coordinate system used, we will have to right click the layer in ArcCatalog => Properties as shown in figure 16 below.
Figure 16: Specifying the coordinate system
After we have clicked on the properties option, we will be provided with the raster dataset properties as shown in figure 17 below.
Figure 17: Raster dataset properties
To specify the spatial reference of the data, we will click on the edit button. In the case of this project, the settings will be made as illustrated by figure 18 below.
Figure 18: Specifying the coordinate system of the file
We will finish the process by clicking the OK button and thereafter the apply button.
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