Assignment 4 Understanding Map Projections and Vertical Datums
Name:______
Introduction:
Managing the projections, datums, and transformations of spatial data is one of the most misunderstood, confusing, and frustrating aspects of GIS. If you have not already encountered problems with data alignment, you will, soon enough. Typically, datasets with projection problems display well away from the intended location, while datum or transformation problems are more likely to show a slight shift (or offset) in space. This lab will increase your familiarity with the types of problems encountered with spatial data, and provide you with the skill necessary to solve projection problems on your own.
Learning Outcomes:
1. Know how to identify a dataset’s projection, datum, and coordinate system.
2. Understand when and why spatial data problems occur.
3. Know the difference between defining a projection and changing a projection.
4. Learn how to solve projection problems through critical thinking.
5. Geoprocessing using the Model Builder interface.
Part 1:
Data Management
1) CREATE YOUR LAB 4 PROJECT WORKSPACE USING THE MODEL BUILDER TOOL
2) Save this Assignment 4 document into your Lab4| Documents folder.
3) Download the zip file Lab4_Data.zip from Blackboard, and extract from the zip file the Lab4_Data folder and save it into your Lab4| Vectors folder. The files stored in Lab4_Data are all vector shapefiles,
Part 2:
Questions
Q1: In your own works, define in a sentence or two the following terms:
Projection:
Datum:
Transformation (hint, see transformation parameters, definition of in ArcGIS help):
Open a new .mxd in ArcMap, add C:\GISdata\BHAM_Subset\AirPhoto02\bham_air-ph_02.sid to your new mxd. Mouse over Layers in the table of contents and right click to view the data frame properties. Select the Coordinate Systems tab.
Q2: What is the current coordinate system datum and Map Units of the layers data frame?
Q3: What are the coordinate system, datum, and projection of the bham_air-ph_02.sid file?
Q4: Where did ArcMap get the coordinate system that it used in the data frame?
Q5: What if the bham_air-ph_02.sid coordinate system was undefined? What coordinate system would the data frame use? Could this be a potential problem? If so, why?
Add the following shapefiles located in …\ Lab4\Vectors\Lab4_Data\Lab4_Projections: SehomeHill1, SehomeHill2, SehomeHill3, SehomeHill4, SehomeHill5, andSehomeHill6 accepting any defaults for transformation or coordinate systems message boxes you receive.
These six files each show the location of the summit of Sehome Hill. Zoom in and confirm that all six points draw on top of each other by turning on and off the layers one-by-one.
View the properties of each Sehomehill shapefile (Properties, Source tab). Fill in values for Coordinate System, Datum, and Projection in Table 1.
Using Windows Explorer (NOT ArcCatalog), navigate to …\ Lab4\Vectors\Lab4_Data\Lab4_Projections. Using Notepad, open the .prj file associated with each of the shapefiles. Summarize the text in the .prj text column as shown in SehomeHill1. For example, open SehomeHill2.prj using Notepad or Wordpad. Do not make any changes to the .prj files, close them without saving.
Table 1
File name / Coordinate System / Datum / Projection / .PRJ textSehomeHill1 / NAD_1983_UTM_Zone_10N / D_North_American_1983 / Transverse_Mercator /
Datum = NAD 1983
Unit = Degree
Projection = Transverse Mercator
False Easting = 5,000,000
False Northing = 0
Central Meridian = -123
Standard Parallel 1 = NA
Standard Parallel 2 = NA
Unit = Meter
SehomeHill2 / NAD_1927_UTM_Zone_10N / D_North_American_1927 / Transverse_Mercator / Datum =
Unit =
Projection =
False Easting =
False Northing =
Central Meridian =
Standard Parallel 1 =
Standard Parallel 2 =
Unit =
SehomeHill3 / Etc.
SehomeHill4
SehomeHill5
SehomeHill6
Q6: If each shapefile uses a different spatial reference system, how does ArcGIS know to display the data sets in the same location given what you know about the data frame projection?
Q7: One of the shapefiles is undefined, yet it still draws correctly on Sehome Hill. Why is this? Explain the relationship between the .prj file and the coordinated system as displayed in ArcMap. Do you always need to have data with defined projections? Can you think of an example where a defined projection is not needed?
Right click Layers in the table of contents, select Properties, and choose the Coordinate systems tab. Select a new coordinate system by expanding predefined, Geographic Coordinate Systems, World, and select WGS 1984. Hit OK and choose OK for any warning messages.
Q8: Knowing that ArcGIS does projection on the fly, explain why some of the points have shifted? Is the shift in these data sets due to projection error, inappropriate default transformations, or God’s will to make you suffer?
Select Sehomehill6 in the table of contents. Right click the file and choose zoom to layer.
Q9: Is the SehomeHill6 point still near the other Sehome Hill points? Where did it go, why? (Hint, mouse over the SehomeHill 6 point and read the coordinates (in “Lat/Long”) from the lower right portion of the screen.)
Reset Layers back to the original coordinate system (See Q.2 above). Hit OK for any warning messages. Confirm that the six shapefiles are again stacked on each other over Sehome Hill. Mouse over the Sehome Hill points.
Q10: What are the coordinates for the summit of Sehome Hill in WA State Plane North, NAD27?
Create a New Model in Model Builder:
Mouse over your ENVS420 ArcTools Toolbox, right click, select new, new model. Expand the Data Management Tools, Projections, and Transformations, Feature toolsets. Drag-and-drop the Project tool onto your new model. From the table of contents, drag-and-drop Sehomehill1 onto the Project box of your model. Double click the Project box to expose the tools parameters. The input data should already be set to your SehomeHill1 file. Set the Output to your Lab4 geodatabase.
Select an Output Coordinate System in a different datum of your choice, for example NAD27WaSPN to NAD83UTM10N, you will need to set at least one transformation file when converting between different datums. Select an appropriate transformation from the the Geographic Transformation drop-down box, if the software does not provide you a default transformation automatically. At this point, you might need to guess, but a real spatial analyst would refer to J:\saldata\ESRI_misc\esri_books\geographic_transformations.pdf. and figure out the best transformation for the area. Under the Model drop-down menu, select Run. Your model should work, if so, congratulations, you just converted some data between two different coordinate systems and datums. If you add this new layer to your map, it may line up with Sehome Hill (if you picked the best transformation file). If it does not align with the other Sehome Hill points, you picked the wrong transformation file :(.
Tools like Project, and Project Raster convert datasets from one known coordinate system/datum to another. The Define Projection tool located in the ArcToolbox Projections and Transformation toolbox are used to assign coordinate system/datum to datasets that do not have them. In essence, Define Projection creates a .prj file for a shapefile that does not have one.
Q11: If you have a shapefile that has an ‘undefined’ projection and an .mxd data frame in UTM, NAD-83, can you use the Define Projection tool to assign this projection to the shapefile? Why or why not?
Use ArcCatalog to make two copies of the undefined SehomeHill file. Rename them testdata1 and testdata2 and save them into your Lab4 geodatabase. Use the Define Projection tool to assign two different projections of your choice to the new files. Now add testdata1 and testdata2 your map. Do testdat1 and testdata2 align on the summit of Sehome Hill? If one of files aligns to Sehome Hill, it is because you got lucky and picked the correct projection that matches the coordinate values of the data. Most likely the two files do not align with Sehome Hill, you have just corrupted your data! Don’t ever use this tool on any data unless you can correctly answer question 12.
Q12: Before using the Define Projection tool, what do you need to know about the data?
In your ModelBuilder model, select the components of your old Project tool and delete them. In ArcToolbox, expand Data Management Tools, Features, and add the tool Add XY Coordinates to your model. Repeat this step five more times. You should have a model with six different Add XY coordinate tools. Set the each of the Add XY Coordinates tools to each of the six SehomeHill shapefiles. Run your model. Open the attribute tables of these file and notice that each attribute table now has two new attributes, Point_X and Point_Y. Record the x and y values in Table 2
Table 2
File name / Point_X / Point_YSehomeHill1
SehomeHill2
SehomeHill3
SehomeHill4
SehomeHill5
SehomeHill6
Q13: Are the values of Point_X and Point_Y the same for each shapefile? If not, what do these coordinates represent? Why are they different? Where does the Add XY Coordinates tool get the x and y values that were inserted into the attribute table?
Making undefined- data draw in the correct location is a common GIS challenge. The next portion of this lab will draw on your experiences so far and test your ability to problem solve a set of troublesome data.
Open a NEW .mxd in ArcMap, add C:\GISdata\BHAM\AirPhoto02\bham_air-ph_02.sid to your new mxd. Add all the shapefile datasets from …\Lab4\Vectors\Lab4_NoProjections. The files are named SehomehillA – SehomeHillE. SehomeHillA – SehomeHillE are the same files from the previous exercise but their .prj files were deleted, thereby removing any useful coordinate information from the data that might be used by ArcGIS to do projection on the fly. Notice that the file points do not draw on top of Sehome Hill. Your task will be to figure out how to get them to draw over the summit of Sehome Hill.
Use the information, tool, or skills gained from the beginning of this lab to complete Table 3. Your answers in table 3 should match the answers entered in table 1.
Some hints to completing table 3 are:
-Change the symbology of the the five point files to ensure you do not confuse the data.
Next, you might...
-Recycle your model builder model for Add x y data. Compare and contrast attributes.
-Change the data frame coordinate system to match those listed in table 1. If one of the points draws on Sehome Hill what can you conclude?
Table 3
File name / Coordinate System / Datum / ProjectionSehomeHillA
SehomeHillB
SehomeHillC
SehomeHillD
SehomeHillE
Finally, pick ONE of the files listed in table 3., use the Define Projection tool, to assign the file the correct projection. Save the file to your Lab4 geodatabase. Name the file: YourLastName_MySehome .
103012 -- Update to Assignment to include Vertical Datums:
Data Management:
1) Download the zip file DOS_Data.zip from Blackboard, and extract from the zip file the DOS_Data folder and save it into your Lab4| Vectors folder. The file stored in DOS_Data is a geodatabase with raster datasets of the Dosewallips Tidal Area.
2) Load Dosewallips_MLLW, Dosewallips_NAVD88, and Dosewallips_HS. Use transparencies and overlay elevation data onto the hillshade.
3) Load an ArcGIS Basemap for to reference the area.
Questions:
Q14. What is the universal elevation offset (difference) between the North American Vertical Datum 1988 (NAVD88) and the Mean Lower Low Water (MLLW) Vertical Datum for tthe Dosewallips Tidal Area? Report your answer in meters.
Q15. High tide in meters when referenced to MLLW is 4.111 meters. What is high tide in meters when referenced to NAVD88?
Q16. How many acres in the Dosewallips are inundated between 0 meters elevation and High Tide when referenced to MLLW? NAVD88? Is there a difference? Why or why not?
Part 3:
Deliverables:
1. This document with the questions answered. Some of the questions have multiple parts; please address each question completely.
2. Hand in your Lab4 geodatabase with that files that were to be saved into it through the lab.