Adding Projection Data to jpegs
Adding Projection Data to jpegs
Often you will have data of an area of interest that has been scanned.
It may
have geographic location information, but this information is not part
of the
digital file, it is only on paper. How do you add this information to
the jpg
so that you can use it with other GIS files and have it related
spatially? That
is what you will learn in this exercise.
Download the data from the this link: Data
It should include the following coverages:
You have a file called Block14.jpg and we would like to give it real
coordinates. We will do this through a process called Georectification.
Open ArcCatalog and drag and drop all of the files into ArcMap. Drag the cntry02 or any of the coverages first so that the numbers in the window in the lower right show latitude and longitude in decimal degrees. Then drag the block14.jpg.
The cntry is a shapefile of the world and is only
for you to
see where the coverages fall. They are provinces in
Now add the jpeg Block 14.jpg You will notice that they are in different areas spatially. We have to give the jpeg geographic coordinates in order for the files to share the same space spatially.
To access the Georeferencing toolbar, we must make it active.
Go to the View icon and then down to Toolbars/Georeferencing – click and it will make it active and put in the View frame. You will be zooming in and out going between the world map in Lat. and Long. and the jpeg that has no coordinates. When you add them all to the window, you should see the world up in the left hand corner and quite small. The Block14 jpeg will be large. Pick an area that you can recognize on both the jpeg and the map. Now on the georeferencing toolbar, select the tool that has a green and red (x). You will select a point on the Block14 jpeg and then locate the same point on the polygons. After about 2 points, it will automatically fall in place. Two to three points is usually enough, but you can select several points until you are statisfied that the fit is a good one. If you do not like any of the points selected, you can click on the icon on the far right which is the table icon and select the last point added and click on the delete button. An example of how it should look, add the following to a new View window: Wintershall.jpg (Save your project as a map document (.mxd) so you can come back to it).
This method is not the
most
accurate, but it will allow you to view data with spatial reference to
other
layers.
To complete the process, once you have the Block14.jpg in the right
geographic
space, go back to the Georeferencing Toolbar and click
on Rectify.
A window will pop up that will allow you to see the pixel size
and the
re-sampling method. Go with the default. The pixel size, unless you
know
exactly what you want that to be, should be left alone. You can
experiment and
see how it changes the image. The re-sample method is a mathematical
algorithm
that processes the image to acquire the geographic information. Nearest
neighbor is commonly used. In the last box, you can browse to a
location where
you want the new file to reside and give it a unique name, or go with
the
default.
To finish the lab, open a new window or view, add
the newly rectified Block14 and either the world map
or the
shapefiles of the Brazilian provinces. The new Block14 should fit
fairly well with the world map or the province
boundaries and you should see decimal degrees in the window at the
bottom right. I want you to create a map with
a north
arrow, scale bar, title (Label it – Lab
7) and name. Export the map and send it as an
attachment.