L2 Projections

7/31/2019 L2 Projections

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Lesson 2: Projections

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Lesson 2: Projecting Geographic Data

WhatyoullLearn: Here we practice map projections and datum transformations in

ArcMap. The readings in Chapter 3, Map Projections and Coordinate Systems, of thetextbook GIS Fundamentals, provide the necessary background.

Dataincludes Minnesota county boundary shapefiles and a lakes dataset, both invarious projections.

What Youll Produce: A map of Minnesota in three different statewide projections, amap of reprojected county boundary and lakes data in central Minnesota, and aworksheet 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 typesof distortion, and we choose the projection which limits distortions to levels we canaccept. Different map projections represent the same point with different X and Y (or Eand N) coordinate values. We cannot mix map projections in an analysis, so we oftenhave 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 (seelast weeks lesson or the video Data Frames for instructions)

Activatethe Albers 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-clickon New York.

The distance between the two cites is displayed, either in a drop-down window, or at thebottom 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


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ArcToolBoxButton

Cursorcoordinates

approximately 3,127 miles.

The on the ground distance between LA and NY is actually 2,444 miles. Thedifference in measurements between the Albers and Mercator is due to unavoidabledistortion caused when we stretch measurements from the curved Earth surface to a flat

map surface.

Projecting ShapefilesYou often need to project data from one coordinate system to a different coordinatesystem. We will performthree different projections,and produce one mapillustrating the differencesbetween the separateprojections. We will also

look at the resultingdifferences in themeasured area for onefeature (in our case acounty) in each projection.

Start ArcMap, create anew empty map, andrename your data framefrom Layers toMinnesota Counties.

Place the L2\minn_county.shpfile in your data frame. You should see a county map ofMinnesota 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 thoseshown.

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 mapprojected coordinate values corresponding to the cursor position. These data in the

minn_county shapefile are in UTM NAD83 projection. Each coordinate value ismeasured in meters, so a value X = 512,349 indicates an X value of 512,349 meters tothe east of the origin.

Note that most data layers have information stored that identifies the appropriatecoordinate system. For example, the data set above is stored in the UTM, NAD83 Zone15 coordinates. I might have another data set of the Minnesota county boundarieswhich is stored in geographic coordinates (latitude/longitude), or in state planeMinnesota South Zone coordinates, or another in an Albers coordinate system. I may


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convert one data set to another through a projection. If you reproject these data layerscorrectly, they will align properly.

Note that you have the option of creating a permanent reprojection or a temporaryreprojection 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 saveyourself much confusion and grief.

Data Frame Coordinate SystemsArcGIS allows a data frame to have a coordinate system. Any data that is suitablydocumented and is subsequently placed in a data frame is converted to that dataframes coordinate system on the fly. This means the coordinate projection is appliedto the data read from the disk, but before it is displayed. This projection is temporary inthat it doesnt 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 datasets 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 sy stem forthe data frame is undefined. If you do not explicitly set the coordinate system for thedata frame, it then takes the coordinate system of the first data set displayed in theframe. All subsequent data are then displayed in this first coordinate system, unlessyou manually override this data frame coordinate system. A few examples will clarifythis. You might also want to look at the video, Intro to Projections.

Create a New Map; (File New, Blank Map, noneed to save your existing map; OK)

Create three new data frames, using the Insert Data Frame operation in the main toolbar (see atright).

Examine the Inserted three separate data frames,note that each data frame is given a name, typicallythe imaginative New Data Frame andNewData Frame 2 etc.

As described in previous Labs, you may activate a data frame by right clicking the namein the table of contents window and selecting Activate, near the bottom of thedropdown menu.

Select the data frame called Layers, which is the first one on your table of contents.


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Note that you may also look at the properties ofthe data frame by left clicking on the data framename to select it, then right clicking andselecting Properties, from the bottom of thedrop-down menu. This will display the Data

Frame Properties window:

In Data Frame Properties there are severaltabs. The most important one for this Lab is theCoordinate System tab. If you select it youshould see the display shown on above right.Notice that this shows the current coordinatesystem, in this case No projection. Notice inthe sub-window on the bottom we may select acoordinate system.Left click, in order, in the Select a coordinate

system: window, on: Predefined

Geographic Coordinate Systems

North America

NAD 1983

Apply

O.K.From now on until you change this, ArcGIS will attempt to convert any data set youdisplay in this frame into the NAD83 Geographic coordinate system.

Add the data layer L2\minn_countyto the Layers data frame. Look at the data layerminn_county. The data on the hard drive is in UTM coordinates, but these coordinates

are converted to Geographic (latitude/longitude) NAD83 before displaying. Move yourcursor about the data, and note the coordinate values in the lower right corner of theframe 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, whichwas then adopted the UTM coordinate system.

Make one of the remaining empty data frames active (remember, select in the table ofcontents, right click, and Activate). Notice the previous map disappears.

For this empty data frame, assign a coordinate system, following the same process asabove, but this time choosing an Albers Equal Area Conic projection, with:


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Properties Coordinate System Tab Predefined Projected CoordinateSystems Continental North America North America Albers Equal AreaConic Apply and OK.

Now, add the minn_county.shpdata to this layer. Note these are projected on the fly to

the new coordinate system. Again, note the differences in the coordinate values forlocations in the state.

Remember, the data on hard drive (or flash drive) are still in the UTM Zone 15coordinates. The data have just been temporarily reprojected to an Albers Equal AreaConic projection for display.

Note that you may get a message when you display a data set that says the datumsmay be incompatible, typically because there is not a datum transformation specified.You may ignore these warnings for this exercise (ONLY!). Datum transformations aredescribed 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 coordinatevalues temporarily. This is often the case when you want to work with disparate datasets occasionally. However, we often want to permanently project the coordinatevalues 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 projectiontool, 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 thecoordinate system written in a file. The projection tool reads this coordinate system todetermine the input. We then specify the output, including the datum transformation, ifneeded, and save the new file to a target location. Be careful to note where you savethe output file, and remember, you are not modifying the original input file; it is still intactin its original location. If the source data set does not contain the identity of its projectedcoordinate system, we must either modify it to include the coordinate system name, orspecify it during projection process.


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Projecting Data to a New File The Project Tool

Create a New Map (FILE New Blank Map) and againinsert two new empty dataframes, then left click to openArcToolbox

Then, left click to select DataManagement Tools Projections andTransformations Feature Project

(Video: Projection Tool)The Project tool steps you

through the process ofprojecting a data layer from onecoordinate 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 (rightclick on data, thenremove). Then for all frames, Set the projection to No Projection by opening theframe properties (right click on the name in the table of contents, then properties), and left clicking Clearin the coordinate system tab, then left clickingApply, thenOK.

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 notaltered)

Select a place to put the new (projected) layer.

Select the projection parameters (they might require both projection andgeographic parameters). These parameters can be loaded from another layeralready in you new projection or you can create a new set of parameters.

Apply the projection parameters If necessary, specify a geographic transformation (also called Datum

Transformations in the text book) Apply the projection

As a useful bit of background information, ArcGIS shapefiles store information about theprojection in a .prj file. For example, a layer named minn_countymay have projectioninformation 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 ofthe projection system, so when you get the unknown projection warning, it is oftenbecause the .prj file is missing.

Option:Open

ArcToolboxSelect DataManagementTools

SelectProjections and

Transformations

Select Project


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Browse to the layer youwish to projectL2\minn_county.shp

Now navigate to whereyou want to store thenew (projected) file.

Name the file; you dontneed to type in the .shplast name.

For this 1st

step namethe new fileminn_county_albers

Detailed InstructionsThis document will step you through the projection screens. You will have to use thesesteps at least six (6) more times in this Lab. In later iterations, refer back to thissequence.

Start the Project Tool from theArcToolbox


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Now you must Select orImport parameters for

the target projection.

Often it is easierImporting from anotherlayer that has theprojection you want, buthere well manuallyselect the projection.

For this 1st

step push theSelect button

A browse menuopens to allow youto define a theoutput coordinatesystem

Select the buttonTo the right of theOutput CoordinateSystem entry line

This opens awindow were youset up the newprojectioninformation

This opens theSpatialReferenceProperties Box


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Left click ProjectedCoordinate Systems thenAdd

Left click Continental thenAdd

Left click North America

then Add


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Left click USA ContiguousAlbers Equal AreaConic.prj

then Add

Left click on Apply and thenOK


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This will create the new data file, projecting from the original coordinate system (a UTMzone 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 framename 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 isnt among those provided

by ArcToolbox. Fortunately, you can create customized coordinate systems, as wellnow do.

Left click on Ok

At times you may need toalso select a GeographicTransformation.Lecture and the GISFundamentals book describegeographic (also calledDatum) transformations, andif required, you should knowthe correct one to use.Otherwise, stop theprojection process, and findout before you proceed.

For this part of the exerciseyou do not need to specify a

geographic transformation(but you will in a subsequentreprojection).


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Create a Custom Projection(Video: Custom Projection)Create or activate a new data frame

Start the Project Tool in ArcToolbox

Select minn_county.shpas the layerto reproject

Name the new fileminn_county_custom_mercator

Click on the Select button to choosea predefined coordinate system foroutput

On the new menu window

Choose Select > ProjectedCoordinate Systems > World >Mercator (world), select Add, thenApply and click OK.

Now select Modify

Insert the cursor at the central meridianvalue, and change it to -93.

Select apply, OK (to close this menu), then Apply, and OK to perform the projectionand close the second menu.

This returns you to the main Project menu.


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Select the geographictransformation forNAD_83_to_WGS_1984_1, thenselect O.K.

This adds yourminn_county_custom_mercatorprojected layer to the active,empty data frame.

Change the name of this data

frame to Custom Mercator (rightclick 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 dataframes (e.g. Minnesota Counties Mercator). Remember to do this by right clicking onthe 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_countyto this data frame.

Open the attribute table for the minn_county layer (right click on the layer and selectOpen Attribute Table, see the Video: Calculate Areas) and notice the values underthe heading Area. Area is the size of the polygon in the map units, in our case


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square meters. Note that value for Area is not updated automatically when youreproject, so you must manually calculate the areas. Well create a new column, andcalculate the area values and place them in this column. We want to record our valuesin square kilometers.

To calculate areas, do the following four steps:

FirstAfter opening thetable, left click onTable OptionsthenAdd Field

Second: Name the field, Sq_km,select Float as the Type:Enter 17 for Precision and 5 forScale, and then left click on OK

Third : Right click over the new Sq_km column to bring up amenu,then right click on Calculate Geometry.

Click OKto the Outside an Edit Session warning.

Finally: In the displayed window,use the Property drop downto select Area and the Units dropdown to select Square Kilometers

Left click on OK


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Record the area in square kilometers, to two decimal places, for St. Louis County, onthe worksheet (use the Word file included with the L2 data ). This is the largest county inMinnesota, in the northeast part of the state. This should be reported in the table in thecolumn you just created. Record and compare the area for St. Louis County under thethree 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 absoluteareas different for each county, but notice how the general shape of the state changeswith each projection. Youll be producing a map with all three views on the layout (seethe example near the end of this lab). You will use a fixed scale to compare these threemaps.

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 ineach data view for easy identification.

On the layout view, select each data frame (one at a time), right click, selectProperties 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. Noteyou shouldnt 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 butany scale that will fit the three maps on apage is fine.


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Your map should look something like the figure below. Note the relative sizedifferences for the respective projections.

Note: ChangeFile Page & Print Setupto Landscape. Data Frame outlines cannot overlap or they willobscure adjacent data. Also Data Frame outlines can be turned off using Data Frame PropertiesFrame Borders none.


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UTM and Various State plane Coordinate Systems

Create a new map by selecting File > New in the main ArcMap Frame, and addL2\minn_count_dd.shpdata layer. This is a data layer of Minnesota county boundariesin 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 cornerof RamseyCounty

Coordinates

Decimal Degrees are different than DegreesMinutes Seconds. You may have to change yourdisplay units (right click on the data frame name,then left click on Properties, General tab, andselect the Decimal Degree units for Display, see atright). Note: County Names can be displayed with theLabelFeaturesoption on the Properties of the Data Layer.


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You can see the coordinates in the lower right corner of the main ArcMap window; x isthe East-West Coordinate, y the North-South Coordinate. Move the cursor over to thenortheast corner of Ramsey County, zooming in as needed, and record the cornerlocation. 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 theminn_count_ddlayer into each of the new data frames using one of three differentcoordinate systems: UTM NAD27, Minnesota State Plane South Zone 1983 (feet), andMinnesota State Plane Central Zone 1983( feet). Use the instructions from theprevious exercise as a guide, and the notes on the next page for specific values toselect for each projection. Add the reprojected data to each frame, and rename eachframe 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 answerswith your .pdf Maps when you complete the lab. Note that the coordinate values forthis same point should be different in different projections. Look at the difference inthe Minnesota Central and Minnesota South State Plane coordinates for the NE cornerof 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:

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5 6

If required select the 1st

Geographic Transformation in


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Project minn_count_dd.shpto the MN State Plane Central, NAD83 projection1) 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 datumtransformation, then OK.

1

Remember units are Feet


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Now, do a similar reprojection to the one above, with the minn_count_dd.shpas input,however, in this case the output target should be the Minnesota State Plane South-NAD1983 coordinate system. We wont 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 tomatch the data sets.

Your ArcMap project should appear similar to the figure below after you have addedyour data layers to the correct data frames.

Examine and record the coordinates for the northeast corner of Ramsey County, nearthe arrow in the figure.


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We will do one final projection, to convert data from state plane coordinates to UTMcoordinates, and display them with data already in our target projection.

Create a new ArcMap project (File > New) and display the following two files in thesame data frame

First add L2\minn_county.shp MN CountiesThen 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 datacontained in the two layers.

Hlakes_not_projectedcontains lake boundaries data in northwestern WashingtonCounty, near the northeast corner of Ramsey County. Note that when displayed with theMinnesota 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 becausethere are different units, different projection shapes and origins, and so the data areprojected to a different set of coordinates. This illustrates why you need to be careful innot mixing data with different projections.

Now remove the Hlakes_not_projectedfrom your ArcMap data frame.

The Hlakes_sp_projecteddata are in the NAD27,Minnesota State Planecoordinate system.

Reproject these to UTMNAD83, Zone 15Ncoordinates, and add thenew reprojected layer to thedata frame with minn_county

Rearrange/recolor the datalayers so you can see boththe county and lakeboundaries. Notice the new;correct locations for these

lake boundariesnear thenortheast corner of RamseyCounty (please see thefigure to the right). Note:County Names can be displayedwith the Label Features option onthe Properties of the Data Layer.


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The second map should look something like this:


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Name:_________________________

Area of St Louis County, Square Kilometers:

UTM zone 15:

Albers:

Custom Mercator:

Coordinates of northeast corner of Ramsey County:

Projection x-coordinate y-coordinate

Decimal degrees

UTM zone 15 (meters)

Minnesota South State

Plane(feet)Minnesota Central StatePlane(feet)

In what state is the origin for the Minnesota South State Plane Zone? (Extra credit):

Documents

L2 Projections