introduction to georeferenced images using arcgis

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Introduction to Georeferenced Images using ArcGIS

Introduction

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. 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.

Georeferenced Images using ArcView 9.x

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:

http://www.library.carleton.ca/madgic/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


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.


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.


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

program.

If there is not a prj file

associated with the image

the projection will have to

be set manually.


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

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.


Coordinate System used for some

of the local area files

Zoom in or out and Select the Area Required


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

area required

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.

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.


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


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

File Menu:

View -> Toolbars -> Effects


rasterimages.pub 25/11/03

Transparency set at 48%

introduction to georeferenced images using arcgis

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