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Spatial
Referencing - Exercise 3
Scenario:
Your task is to review
disparate data sources for an area
through these exercises. These data will show the 2 tank
farms in Bucksport and their location relative to Penobscot River at a
scale of approximately 1:10,000. Such data would be used for
planning purposes like siting of expanded landfill area. Data
will include basic raster data such as aerial photos, basic vector data
such as roads, and additional CAD files, scanned maps, and
shapefiles. In this exercise, you will look at how the data are
referenced spatially, see how to reproject data, and how to
georeference a scanned-in map.
Step 1 Spatial Referencing
All features in a GIS are spatially referenced, that is each feature is
assigned coordinates on a coordinate plane which are related to a
consistent grid system. In most cases at MEDEP, this means the
data are in the UTM/NAD83 coordinate system.
In ArcMap, switch to the Layout View
and look at the grid. This grid is a UTM grid with intervals
every 100 meters. Use the 'Identify' 
tool and click on one of the sample points  . You will see the
coordinates shown at the top of the 'Identify' results (the coordinates
of where you actually clicked) and in two fields associated with the
layer UTM_X and UTM_Y. Note the relationship of these coordinates
to the grid lines.
1.) Do the coordinates returned from the 'Identify' tool line up
with the grid?_____ Why or why not?__________________________
Step
2 Coordinate systems
Every
GIS dataset (feature class) uses spatial referencing to assign a
coordinate system to it. The coordinate system of the data may
also be known as the data projection. In some cases, this may be
undefined, which only means the coordinate system is not known (but it
still exists).
Right-click any of the layers in your ArcMap Table of Contents and
bring up its layer properties. Click on the 'Source' tab.
You will see the coordinate system of the layer.
2.) What coordinate system is used for each layer?
Sample_Location_BRWM_and_EGAD _______________________
Contours_BRWM ________________________________________
Structure_BRWM ________________________________________
USGS_DOQ_Orthophotos_96_98 ___________________________
Step
3 Map Projections
The terms "map
projection" and "coordinate system" are often used interchangeably, but
there is a subtle difference. The coordinate system is used as a
reference grid for spatial data. A map projection is a method of
projecting a 3-D surface onto a flat plane for representation in a
map. At MEDEP, we typically use the UTM coordinate system as both
our coordinate system and our map projection for maps. A map
projection can be changed without changing the coordinate system of the
data.
In ArcMap Table of Contents, right-click the "Layers" data
frame  and bring up its properties.
Click on the 'Coordinate System' tab.
3.) What map projection (coordinate system) is currently in use?
_______________________________________
Select a new coordinate system for the map under Predefined -
Geographic Coordinate Systems - North America - North American Datum
1983. This means the map will be displayed in lat/long.
Click 'Apply' to make the change.
4.) Did this change the way the map looks? _______ Why or why
not?_______________________________________
5.) If latitude/longitude is so widely used, why don't we just
use that for all our coordinate systems? Why mess around with UTM?
___________________________________________________________________________________________________________
Change the Coordinate System back to UTM (look under 'Layers' on the
'Coordinate System' tab and choose one of the layers above which uses
UTM NAD83).
Step
4 Georeferencing
In this step,
you will take an unregistered data file (such as a scanned-in map) and
align it to known GIS features. This is called georeferencing.
In ArcMap, zoom to the Webber tank farm and switch to Data View.
Add the file 'bucksport_webber.tif' from your training folder.
6.) Why don't you see the file after you add it?
_____________________________________________________
Use the 'Identify' tool
and click any feature in the
map. Record the coordinates ___________________
Right-click the new layer and choose 'Zoom to Layer'.
7.) Why don't you see your other data now?
_______________________________________________________
Use the 'Identify'
tool and click any feature in the
map.
8.) What does the difference in coordinates indicate?
___________________________________________________
9.) Does this file contain raster or vector data? ________
Is it CAD data?________________
Based on your answers to # 9 above, activate the correct georeferencing
toolbar.
10.) What toolbar will you use?_________________________
Make the new data file 50% transparent so you can
see the other data behind it Now use the 'webber_farm' bookmark
under View-Bookmarks to display the tank farm. On your toolbar,
click 'Fit to
Display'. That will force the file to draw in the same coordinate
space as your other data.
Start by rotating  the file until it lines up pretty
well. Then use the links  tool to create at least three
links between features in the new file and their corresponding
locations in your map. As you add each link, the image will move
to fit. If you don't like one of your links, you can view the
links  and delete the one you want.
You can also see the error each link contributes in the link table (if you have 3 or fewer links,
there will be no error). When you are happy with the fit of the
image, choose 'Update Georeferencing' from the toolbar menu.
11.) Will you need to use rectification? ________ Why or
why not? _______________________________________
Answers
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