Lab3paulbolstad.net/5thedition/Instructions/L2 Projections.docx · Web view2012. 3. 14. · Often...
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Lesson 2: Projections Lesson 2: Projecting Geographic Data What you’ll Learn: Here we practice map projections and datum transformations in ArcMap. The readings in Chapter 3, Map Projections and Coordinate Systems, of the textbook GIS Fundamentals, provide the necessary background. Data includes Minnesota county boundary shapefiles and a lakes dataset, both in various projections. What You’ll Produce: A map of Minnesota in three different statewide projections, a map of reprojected county boundary and lakes data in central Minnesota, and a worksheet recording areas and coordinates for various projections. Background: The Earth's surface complexly curved. We introduce unavoidable distortion when we flatten this curved surface onto a map, typically changing areas, lengths, and the shapes of features. Different map projections introduce different types of distortion, and we choose the projection which limits distortions to levels we can accept. Different map projections represent the same point with different X and Y (or E and N) coordinate values. We cannot mix map projections in an analysis, so we often have to re-project some of our data layers. Observing How Distance Changes with the Map Projection Start ArcMap, and add two data frames. Name one Albers, and the other Mercator (see last week’s lesson or the video Data Frames for instructions) Activate the Alber’s Layer Add the layers twocity_Albers.shp, and USA_48_Albers.shp. Left click on the Measure Tool to enable it, and set the Distance Units to Miles (see Lab 1, or video Measure Tool.mov) 1
Lab3paulbolstad.net/5thedition/Instructions/L2 Projections.docx · Web view2012. 3. 14. · Often it is easier Importing from another layer that has the projection you want, but
Lab3Lesson 2: Projecting Geographic Data
What you’ll Learn: Here we practice map projections and datum
transformations in ArcMap. The readings in Chapter 3, Map
Projections and Coordinate Systems, of the textbook GIS
Fundamentals, provide the necessary background.
Data includes Minnesota county boundary shapefiles and a lakes
dataset, both in various projections.
What You’ll Produce: A map of Minnesota in three different
statewide projections, a map of reprojected county boundary and
lakes data in central Minnesota, and a worksheet recording areas
and coordinates for various projections.
Background: The Earth's surface complexly curved. We introduce
unavoidable distortion when we flatten this curved surface onto a
map, typically changing areas, lengths, and the shapes of features.
Different map projections introduce different types of distortion,
and we choose the projection which limits distortions to levels we
can accept. Different map projections represent the same point with
different X and Y (or E and N) coordinate values. We cannot mix map
projections in an analysis, so we often have to re-project some of
our data layers.
Observing How Distance Changes with the Map Projection
Start ArcMap, and add two data frames. Name one Albers, and the
other Mercator (see last week’s lesson or the video Data Frames for
instructions)
Activate the Alber’s Layer
Add the layers twocity_Albers.shp, and USA_48_Albers.shp.
Left click on the Measure Tool
to enable it, and set the Distance Units to Miles (see Lab 1, or
video Measure Tool.mov)
Left-click once on Los Angeles, then move the mouse to New York and
double left-click on New York.
The distance between the two cites is displayed, either in a
drop-down window, or at the bottom left of the ArcMap window (it
depends on the version and setup).
Your measured distance should be approximately 2,440 miles.
Activate the Mercator data frame. Add the layers
twocity_Mercator.shp, USA_48_Mercator.shp
Re-measure the distance from LA to NY. The new measurement should
be approximately 3,127 miles.
The on the “ground distance” between LA and NY is actually 2,444
miles. The difference in measurements between the “Albers” and
“Mercator” is due to unavoidable distortion caused when we stretch
measurements from the curved Earth surface to a flat map
surface.
Projecting Shapefiles
( ArcToolBox Button Cursor coordinates )
You often need to project data from one coordinate system to a
different coordinate system. We will perform three different
projections, and produce one map illustrating the differences
between the separate projections. We will also look at the
resulting differences in the measured area for one feature (in our
case a county) in each projection.
Start ArcMap, create a new empty map, and rename your data frame
from “Layers” to “Minnesota Counties.”
Place the L2\minn_county.shp file in your data frame. You should
see a county map of Minnesota displayed in your screen, similar to
the figure at above.
Note the location of the ArcToolbox button and the cursor
coordinates. Remember, because the toolbars are moveable, they may
be in different locations than those shown.
Move your cursor around the screen and notice the coordinate values
to the lower right. Note how these change along with the cursor
position as the program displays the map projected coordinate
values corresponding to the cursor position. These data in the
minn_county shapefile are in UTM NAD83 projection. Each coordinate
value is measured in meters, so a value X = 512,349 indicates an X
value of 512,349 meters to the east of the origin.
Note that most data layers have information stored that identifies
the appropriate coordinate system. For example, the data set above
is stored in the UTM, NAD83 Zone 15 coordinates. I might have
another data set of the Minnesota county boundaries which is stored
in geographic coordinates (latitude/longitude), or in state plane
Minnesota South Zone coordinates, or another in an Albers
coordinate system. I may convert one data set to another through a
projection. If you reproject these data layers correctly, they will
align properly.
Note that you have the option of creating a permanent reprojection
or a temporary reprojection with ArcGIS. This can be quite
confusing at first, so read this section carefully, and make sure
you understand it before you go on – you will likely save yourself
much confusion and grief.
Data Frame Coordinate Systems
ArcGIS allows a data frame to have a coordinate system. Any data
that is suitably documented and is subsequently placed in a data
frame is converted to that data frame’s coordinate system “on the
fly”. This means the coordinate projection is applied to the data
read from the disk, but before it is displayed. This projection is
“temporary” in that it doesn’t affect the data stored on the disk –
it only reprojects the data temporarily, for display. This allows
us to display many data sets in a data frame even if the data sets
are stored in different map projections, without having to go to
the trouble of manually reprojecting each data set and saving a new
version of the data set.
The “catch” comes in that when you first create a data frame, the
coordinate system for the data frame is undefined. If you do not
explicitly set the coordinate system for the data frame, it then
takes the coordinate system of the first data set displayed in the
frame. All subsequent data are then displayed in this “first”
coordinate system, unless you manually override this data frame
coordinate system. A few examples will clarify this. You might also
want to look at the video, Intro to Projections.
Create a New Map; (FileNew, “Blank Map”, no need to save your
existing map; OK)
Create three new data frames, using the Insert – Data Frame
operation in the main toolbar (see at right).
Examine the Inserted three separate data frames, note that each
data frame is given a name, typically the imaginative “New Data
Frame” and
“New Data Frame 2” etc.
As described in previous Labs, you may activate a data frame by
right clicking the name in the table of contents window and
selecting Activate, near the bottom of the dropdown menu.
Select the data frame called “Layers”, which is the first one on
your table of contents.
Note that you may also look at the properties of the data frame by
left clicking on the data frame name to select it, then right
clicking and selecting Properties, from the bottom of the drop-down
menu. This will display the Data Frame Properties window:
In Data Frame Properties there are several tabs. The most important
one for this Lab is the Coordinate System tab. If you select it you
should see the display shown on above right. Notice that this shows
the current coordinate system, in this case “No projection”. Notice
in the sub-window on the bottom we may select a coordinate
system.
Left click, in order, in the “Select a coordinate system:” window,
on:
· Predefined
· Apply
· O.K.
From now on until you change this, ArcGIS will attempt to convert
any data set you display in this frame into the NAD83 Geographic
coordinate system.
Add the data layer L2\minn_county to the “Layers” data frame. Look
at the data layer minn_county. The data on the hard drive is in UTM
coordinates, but these coordinates are converted to Geographic
(latitude/longitude) NAD83 before displaying. Move your cursor
about the data, and note the coordinate values in the lower right
corner of the frame – They should be different than for the UTM
(meters) data you observed before, when you first loaded the data
into a data frame with an unspecified projection, which was then
adopted the UTM coordinate system.
Make one of the remaining empty data frames active (remember,
select in the table of contents, right click, and Activate). Notice
the previous map disappears.
For this empty data frame, assign a coordinate system, following
the same process as above, but this time choosing an Alber’s Equal
Area Conic projection, with:
Properties Coordinate System Tab Predefined Projected Coordinate
Systems Continental North America North America Albers Equal Area
Conic Apply and OK.
Now, add the minn_county.shp data to this layer. Note these are
projected “on the fly” to the new coordinate system. Again, note
the differences in the coordinate values for locations in the
state.
Remember, the data on hard drive (or flash drive) are still in the
UTM Zone 15 coordinates. The data have just been temporarily
reprojected to an Albers Equal Area Conic projection for
display.
Note that you may get a message when you display a data set that
says the datums may be incompatible, typically because there is not
a datum transformation specified. You may ignore these warnings for
this exercise (ONLY!). Datum transformations are described in the
textbook, GIS Fundamentals, and elsewhere, and whether datum
transformation differences are important depend on the source and
target projections, the accuracy requirements of the data, and the
goals of the analyses.
The previously described exercise shows how you may reproject the
coordinate values temporarily. This is often the case when you want
to work with disparate data sets occasionally. However, we often
want to permanently project the coordinate values in a data set
from one coordinate system to another. We create a new data set,
projecting from an original source data set to a target data
set.
We accomplish this in ArcGIS with the projection tool. Each time we
apply the projection tool, we identify the source data set, the
output data set, and the output projection. Most source data sets
have a coordinate system associated with them, with the identity of
the coordinate system written in a file. The projection tool reads
this coordinate system to determine the input. We then specify the
output, including the datum transformation, if needed, and save the
new file to a target location. Be careful to note where you save
the output file, and remember, you are not modifying the original
input file; it is still intact in its original location. If the
source data set does not contain the identity of its projected
coordinate system, we must either modify it to include the
coordinate system name, or specify it during projection
process.
Projecting Data to a New File – The Project Tool
( Option: Open ArcToolbox Select Data Management Tools Select
Projections and Transformations Select Project )
Create a New Map (FILE NewBlank Map) and again insert two new empty
data frames, then left click to open ArcToolbox
Then, left click to select Data Management Tools Projections and
Transformations Feature Project
(Video: Projection Tool)
The Project tool steps you through the process of projecting a data
layer from one coordinate system to another. There are several
steps.
Note: If you did not create a New Map as directed above, remove any
data from all data frames (right click on data, then remove). Then
for all frames, Set the projection to “No Projection” by opening
the frame properties (right click on the name in the table of
contents, then properties), and left clicking Clear in the
coordinate system tab, then left clicking Apply, then OK.
Detailed instructions for specific projection examples are provided
a bit further on in this document; here we outline the general
process: (read on for specific step by step instructions)
· Create a new data frame, or Activate a data frame with no
projection assigned
· Start the projection tool.
· Select a starting layer (the one you wish to project; you create
a new layer, the original is not altered)
· Select a place to put the new (projected) layer.
· Select the projection parameters (they might require both
projection and geographic parameters). These parameters can be
loaded from another layer already in you new projection or you can
create a new set of parameters.
· Apply the projection parameters
· Apply the projection
As a useful bit of background information, ArcGIS shapefiles store
information about the projection in a .prj file. For example, a
layer named minn_county may have projection information stored in
the file minn_county.prj. The .prj file is not mandatory, however,
even though all data do have a coordinate system. Without a .prj,
ArcMap is ignorant of the projection system, so when you get the
unknown projection warning, it is often because the .prj file is
missing.
Detailed Instructions
This document will step you through the projection screens. You
will have to use these steps at least six (6) more times in this
Lab. In later iterations, refer back to this sequence.
( Browse to the l ayer you wish to project L2\minn_county.shp Now
navigate to where you want to store the new (projected) file. Name
the file; you don’t need to type in the . shp last name. For this 1
st step name the new file minn_county_albers )
Start the Project Tool from the ArcToolbox
( Now you must Select or Import parameters for the target
projection. Often it is easier Importing from another layer that
has the projection you want, but here we’ll manually select the
projection. For this 1 st step push the Select button A b rowse
menu opens to allow you to define a the output c oordinate s ystem
)
( Select the button To the right of the Output Coordinate System
entry line This opens a window were you set up the new projection
information This opens the Spatial Reference Properties Box )
( Left click Projected Coordinate Systems then Add ) ( Left click
Continental then Add ) ( Left click North America then Add )
( Left click “USA Contiguous Albers Equal Area Conic.prj ” then Add
) ( Left click on Apply and then OK )
( Left click on Ok At times you may need to also select a
Geographic Transformation. Lecture and the GIS Fundamentals book
describe geographic (also called Datum) transformations, and if
required, you should know the correct one to use. Otherwise, stop
the projection process, and find out before you proceed. For this
part of the exercise you do not need to specify a geographic
transformation (but you will in a subsequent reprojection ).
)
This will create the new data file, projecting from the original
coordinate system (a UTM zone 15 North system) to the new, Albers
coordinate system you specified.
Now add the newly projected layer to an empty data frame, if it is
not already added.
Change the name of this data frame to Albers (remember, right click
on the data frame name in the table of contents, then select the
Properties option, then the General tab, then type the new name in
the Name textbox).
Sometimes, the target coordinate system you want to use isn’t among
those provided by ArcToolbox. Fortunately, you can create
customized coordinate systems, as we’ll now do. Create a Custom
Projection
(Video: Custom Projection)
· Start the Project Tool in ArcToolbox
· Select minn_county.shp as the layer to reproject
· Name the new file minn_county_custom_mercator
· Click on the Select button to choose a predefined coordinate
system for output
On the new menu window
· Choose Select > Projected Coordinate Systems > World >
Mercator (world), select Add, then Apply and click OK.
Now select Modify
Insert the cursor at the central meridian value, and change it to
-93.
Select apply, OK (to close this menu), then Apply, and OK to
perform the projection and close the second menu.
This returns you to the main Project menu.
Select the geographic transformation for NAD_83_to_WGS_1984_1, then
select O.K.
This adds your minn_county_custom_mercator projected layer to the
active, empty data frame.
Change the name of this data frame to Custom Mercator (right click
on data frame in TOC > Properties > General Tab >
Name).
Change the names of the county data sets to more or less match
those of the data frames (e.g. “Minnesota Counties Mercator”).
Remember to do this by right clicking on the shapefile layer name
in the TOC > Properties > General > Layer Name).
Add a Column and Calculate Areas in an Attribute Table
Create a new EMPTY data frame. (If you already have an empty one,
Activate it)
Rename this frame to UTM NAD1983
Add L2\minn_county to this data frame.
Open the attribute table for the minn_county layer (right click on
the layer and select “Open Attribute Table,” see the Video:
Calculate Areas) and notice the values under the heading “Area”.
“Area” is the size of the polygon in the map units, in our case
square meters. Note that value for “Area” is not updated
automatically when you reproject, so you must manually calculate
the areas. We’ll create a new column, and calculate the area values
and place them in this column. We want to record our values in
square kilometers.
To calculate areas, do the following four steps:
( First After opening the table, left click on Table Options then
Add Field )
( Second: Name the field, Sq_km , s elect Float as the Type: Enter
17 for Precision and 5 for Scale , and then left click on OK
)
( Third : R ight click over the new Sq_km column to bring up a m
enu , then right click on Calculate Geometry . Click OK to the
“Outside an Edit Session” warning. )
( Finally : In the displayed window, use the Property drop down to
s elect Area and the Units drop down to s elect Square Kilometers
Left click on OK )
Record the area in square kilometers, to two decimal places, for
St. Louis County, on the worksheet (use the Word file included with
the L2 data ). This is the largest county in Minnesota, in the
northeast part of the state. This should be reported in the table
in the column you just created. Record and compare the area for St.
Louis County under the three different projections. Two should be
just a bit different, and the third quite different from the
others.
Also note the shape of the state with the different projections.
Not only are the absolute areas different for each county, but
notice how the general shape of the state changes with each
projection. You’ll be producing a map with all three views on the
layout (see the example near the end of this lab). You will use a
fixed scale to compare these three maps.
· First, go to the layout view, and in File > Page Setup, change
to Landscape
· Then reposition the 3 data frame boxes to be side by side and
about the same size (see example near end of lab). You may want to
change the colors of the layers in each data view for easy
identification.
· On the layout view, select each data frame (one at a time), right
click, select Properties and from the Data Frame tab change to a
fixed scale (see figure below).
· Make each the same scale, something near 1:14,500,000 to compare
them. Note you shouldn’t enter the commas, we just show them to
avoid confusion.
Choose the same fixed scale for each data frame and Apply, then
OK.
A scale of 1:14500000 works well but any scale that will fit the
three maps on a page is fine.
Your map should look something like the figure below. Note the
relative size differences for the respective projections.
Note: Change FilePage & Print Setup to Landscape. Data Frame
outlines cannot overlap or they will obscure adjacent data. Also
Data Frame outlines can be turned off using Data Frame Properties
FrameBordersnone.
UTM and Various State plane Coordinate Systems
Create a new map by selecting File > New in the main ArcMap
Frame, and add L2\minn_count_dd.shp data layer. This is a data
layer of Minnesota county boundaries in decimal degrees
coordinates. Record the decimal degree coordinates of the northeast
corner of Ramsey County (see the map figure,
at below) in decimal degrees on the sheet at the end of this
Lab.
NE corner
of Ramsey
County
Coordinates
Decimal Degrees are different than Degrees Minutes Seconds. You may
have to change your display units (right click on the data frame
name, then left click on Properties, General tab, and select the
Decimal Degree units for Display, see at right). Note: County Names
can be displayed with the Label Features option on the Properties
of the Data Layer.
You can see the coordinates in the lower right corner of the main
ArcMap window; x is the East-West Coordinate, y the North-South
Coordinate. Move the cursor over to the northeast corner of Ramsey
County, zooming in as needed, and record the corner location. Make
sure you zoom in until the coordinate changes only to the right of
the decimal point when you move it just off the corner.
Now, create three new DATA FRAMES, then use ArcToolbox to reproject
the minn_count_dd layer into each of the new data frames using one
of three different coordinate systems: UTM NAD27, Minnesota State
Plane South Zone 1983 (feet), and Minnesota State Plane Central
Zone 1983( feet). Use the instructions from the previous exercise
as a guide, and the notes on the next page for specific values to
select for each projection. Add the reprojected data to each frame,
and rename each frame appropriately.
Note the coordinates for the northeast of Ramsey County in each
different projection, and record them in the document
(L2\L2_Data_Sheet.doc) and submit your answers with your .pdf Maps
when you complete the lab. Note that the coordinate values for this
same point should be different in different projections. Look at
the difference in the Minnesota Central and Minnesota South State
Plane coordinates for the NE corner of Ramsey County.
Which state contains the origin (x=0, y=0) for the Minnesota South
State Plane Zone?
Step-by step for reprojection to UTM NAD27 (follow the numbers on
the sides)
For the UTM NAD27 projection, in the projection tool:
1) Select projected coordinates, 2) UTM, 3) NAD27, 4) Zone 15N, 5)
Apply-OK, 6) For the Geographic Transformation, select the first
one in the list of options.
Below are some screens you will see as you step through these
processes:
For the UTM, NAD27:
( 2 )
( 1 )
( 3 )
( 4 )
( 6 If required select the 1 st Geographic Transformation in the
options list )
( 5 )
( Project minn_count_dd.shp to the MN State Plane Central , NAD83
projection Select projected coordinates, 2) State Plane, 3) NAD83
(feet), 4) Minnesota Central, then Apply/OK (not shown) 5) Select
the NAD_83_to_WGS_84_1 datum transformation, then OK. Screen views
for the projection to MN St Plane Central - 1983 (Feet) )
( 5 )
( 4 )
( 2 ) ( 1 )
Now, do a similar reprojection to the one above, with the
minn_count_dd.shp as input, however, in this case the output target
should be the Minnesota State Plane South- NAD1983 coordinate
system. We won’t show the screens here.
Add each data set into a unique data frame, and make sure the data
frame coordinate system matches the shape file coordinate system,
and rename the data frames to match the data sets.
Your ArcMap project should appear similar to the figure below after
you have added your data layers to the correct data frames.
Examine and record the coordinates for the northeast corner of
Ramsey County, near the arrow in the figure.
We will do one final projection, to convert data from state plane
coordinates to UTM coordinates, and display them with data already
in our target projection.
Create a new ArcMap project (File > New) and display the
following two files in the same data frame
First add L2\minn_county.shp MN Counties
Then add L2\hlakes_not_projected.shp Hugo Lakes
(Ignore the warning that hlakes_not_projected is not in the same
coordinates system)
Click on the zoom to full extent button,
and notice the relative location of the data contained in the two
layers.
Hlakes_not_projected contains lake boundaries data in northwestern
Washington County, near the northeast corner of Ramsey County. Note
that when displayed with the Minnesota UTM data, the lakes data
appear as a very small speck of dust, well south and east of
Minnesota. The data are wrongly placed relative to each other
because there are different units, different projection shapes and
origins, and so the data are projected to a different set of
coordinates. This illustrates why you need to be careful in not
mixing data with different projections.
Now remove the Hlakes_not_projected from your ArcMap data
frame.
The Hlakes_sp_projected data are in the NAD27, Minnesota State
Plane coordinate system.
Reproject these to UTM NAD83, Zone 15N coordinates, and add the new
reprojected layer to the data frame with minn_county
Rearrange/recolor the data layers so you can see both the county
and lake boundaries. Notice the new; correct locations for these
lake boundaries…near the northeast corner of Ramsey County (please
see the figure to the right). Note: County Names can be displayed
with the Label Features option on the Properties of the Data
Layer.
The second map should look something like this:
Name:_________________________
UTM zone 15:
Projection
x-coordinate
y-coordinate
(feet)
In what state is the origin for the Minnesota South State Plane
Zone? (Extra credit):
11