Introduction to Georeferenced Images using ArcGIS · PDF file Introduction to Georeferenced...
Transcript of Introduction to Georeferenced Images using ArcGIS · PDF file Introduction to Georeferenced...
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.
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. http://arcscripts.esri.com 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.
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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:
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
Use the Add Data Icon
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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.
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Select the image file and
then click Add
If this dialog box appears
select Build Pyramids See ArcMap online help
for more details if
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In the Table of Contents
to the left of the display
right click on Layers to
display a menu. Select
Properties. Double click.
An Image might come up
with this error message if the
spatial reference or
projection information is
not available for the
If there is not a prj file
associated with the image
the projection will have to
be set manually.
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The Data Frame Properties Dialog appears.
Navigate to the
Coordinate System Tab.
•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
Predefined ->Projected Coordinate Systems->National Grids->Canada-> NAD 1983 MTM9
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.
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Coordinate System used for some
of the local area files
Zoom in or out and Select the Area Required
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The Map Scale is now set
in the image to 1:1,000
Select the Zoom in icon
or the Zoom out icon
and with the mouse draw a
rectangle on the image of the
Exporting a Selected Image as a Georeferenced Image File
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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
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
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 unavail-able 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.
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Select TIFF Options:
4 Dots per inch 4 Make sure that Create World file is selected for a
georefererenced image 4 Click OK
With Some Transparency Original Image More Transparency
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Select the layer in the Layer pulldown dialog of the image
that you want to make
If the Effects Toolbar is not visible
select the Toolbar from the
View -> Toolbars -> Effects
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Transparency set at 48%