What do I do first in ArcView 8.x? When the program starts Select from the Dialog box: A new empty map

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1 Introduction Introduction to Georeferenced Images using ArcGIS Georeferenced images such as aerial photographs or satellite images can be used in many ways in both GIS software and image software. Images can be used as a background for vector files, a transparent overlay, for on-screen digitizing or for analyzing the raster image itself. You might also just want a section of an image to be printed but it might have to be exported to specific scale. Some of the basics are covered in this handout: 4 Setting the projection to be able to have the scale accurately recorded. 4 Exporting a georeferenced section of an image. 4 Setting the transparency of an image. The Library has a large collection of local Ottawa area images including orthophotos at different resolutions from the National Capital Commission, georeferenced air photos from the City of Ottawa, and satellite images from Natural Resources Canada. Definitions * Georeference: The process of defining how raster data is situated in map coordinates. Georeferencing raster data allows it to be viewed, queried, and analyzed with other geographic data. Raster: Represents any data source that uses a grid structure to store geographic information. Image: Represents geographic features by dividing the world into discrete squares called cells. Examples include satellite and aerial photographs, scanned documents, and building photographs. Can I use ArcVIew 3.x to export part of an image? Yes but the instructions in this handout are for ArcGIS 9.x. There is a script, GeoJPEG, on the ESRI website that can be used to export part of an image. GeoJPEP.avx has been downloaded and is available on the GIS Workstation in the Library in the scripts folder. GeoJPEG.avx Summary: Exports a View as a geo-referenced JPEG file. May require the 3D Analyst Extension to enable the Avenue requests used. For more information on GeoJPEG see the website and for information on how to use avenue scripts with ArcView 3.x see the program s help pages or the manual (available in the Library on Reserve). ArcView 3.x does not support transparency in images.

2 Page 2 What do I do first in ArcView 8.x? Start the desktop mapping program ArcGIS (available on the Mapping Workstation in MADGIC). If there is no icon on the desktop go to the Start Menu -> ArcGIS -> ArcMap. You should have already selected the georeferenced image(s) that you want to use. Indexes are available on the GIS website at: maps/gisweb/index.htm Some indexes are also available in paper in MADGIC. When the program starts Select from the Dialog box: A new empty map Select File -> Add Data or Use the Add Data Icon

3 Page 3 Georeferenced Images using ArcView 9.x Navigate to the folder that contains the image file that you require If the Folder or the drive is not present in the Add Data dialog box use the Connect to folder icon and select the drive needed.

4 Page 4 Select the image file and then click Add If this dialog box appears select Build Pyramids See ArcMap online help for more details if required.

5 Page 5 Georeferenced Images using ArcView 9.x An Image might come up with this error message if the spatial reference or projection information is not available for the program. If there is not a prj file associated with the image the projection will have to be set manually. In the Table of Contents to the left of the display right click on Layers to display a menu. Select Properties. Double click.

6 Page 6 The Data Frame Properties Dialog appears. Navigate to the Coordinate System Tab. Choices include: Favorites: For Ottawa area images select NAD 1983 MTM9 Predefined: select the correct coordinate system. For City of Ottawa air photos and NCC orthophotos Navigate to: Predefined ->Projected Coordinate Systems->National Grids->Canada-> NAD 1983 MTM9

7 Page 7 Georeferenced Images using ArcView 9.x Coordinate System used for some of the local area files Scale When the Coordinate System is set the Map Scale will appear on the Toolbar as seen below. The Image can be selected by changing the scale number in the display box above If you know the scale that you need type it into the box or use the Pull down menu for some standard selections. Be careful this is where the resolution of the original raster image file will make a difference - the relationship between distance and pixels in the image. In the National Capital Commission series of orthophotos there is a choice of 1 metre, ½ metre and ¼ meter resolution. The ¼ metre resolution will allow more zooming in without loss of detail. However the smaller the resolution the larger the file size.

8 Page 8 The Map Scale is now set in the image to 1:1,000 Zoom in or out and Select the Area Required Select the Zoom in icon or the Zoom out icon and with the mouse draw a rectangle on the image of the area required

9 Page 9 Georeferenced Images using ArcView 9.x Exporting a Selected Image as a Georeferenced Image File In the Map or Display Window have the image or section of the image that you want to export selected On the File Menu Select Export Map To export a georeferenced file in the Export Dialog box select Save as Type: TIFF for a georeferenced file. If a georeferenced file is not required choose the file format desired.

10 Page 10 Select TIFF Options: 4 Dots per inch 4 Make sure that Create World file is selected for a georefererenced image 4 Click OK Setting the Transparency in an Image Sometimes it is useful to have the raster image transparent or semi-transparent. Using the Effects toolbar in ArcMap different degrees of transparency can be set. Try experimenting with higher and lower transparency values. Transparency is unavailable in ArcView 3.x. With transparency, polygon themes can be positioned beneath a transparent image layer and viewed as though the image was merged into the polygon layers. Original Image With Some Transparency More Transparency

11 Page 11 If the Effects Toolbar is not visible select the Toolbar from the File Menu: View -> Toolbars -> Effects Select the layer in the Layer pulldown dialog of the image that you want to make

12 Page 12 Transparency set at 48% rasterimages.pub 25/11/03