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Assignment 4 - GIS Data Quality Assessment

Format: Text document. Place on Student Work Zone section of the wiki

The goal of this assignment is for you to gain a deeper understanding of existing GIS data layers and to start to learn how to assess the appropriateness of a data set for a given project.

Because we can only assess the "fitness for use" of any given data set based on what a particular project's needs are, you need to think about data quality in terms of those needs. For this assignment, pretend that you are working on a project and need to assess existing data sets in terms of how well they would work for your purposes. You need to define the overall project idea and its accuracy needs, and then answer the data quality questions below in terms of this defined project (note: this will probably be an imaginary project; if you can relate it to your final project, try to do that, but it is not required).

Collect at least 8 GIS data layers (the four required below, plus four optional layers you choose) for the area in which you live or an area with which you are very familiar. This should be at the scale of a few blocks so that you can take a very detailed look at the data and if necessary, go out and visit the area on foot.

See the Tips for Reducing your Data Layers to Just Your Study Area at the end of this page - this will help speed things up a lot.

See the Notes on where to locate the data sets further on in this assignment.

Required data layers

The following data layers are required in your report - see the Data Notes at the end of this page for more information and tips:


  1. Road centerlines from a city or state agency (e.g., MassGIS or City of Boston)
  2. Road centerlines from the Census 2013 Tiger Address Range Feature Shapefile data set - directions to acquire these:
    1. go to  2013 TIGER Shapefiles download site
    2. In the Select Layer box, scroll down to Feature Relationships, then select Relationship Files - click Submit
    3. Go to the option for Address Range Feature Shapefiles (be sure to pick this one and not any other!! See this graphic!) - and select your State
    4. Select your County and click Download
    5. Extract (unzip) using PowerArchiver or another decompression program.

Note: if you have gotten 2012 TIGER roads already, that's fine, you can use those for this assignment.


  1. Hydrography from a city, state, or the National Hydrography Data set (note: I have NHD data for the Charles and Merrimac River Watersheds here - S:\classes\UEP_ENV\GIS Data\National Hydrography Data Examples). You can also get NHD data from the National Map.
  2. Hydrography from the Census TIGER 2012 data set - first select the choice for FeaturesWater - then you can choose Linear (streams) or Area (lakes, ponds, rivers) features or both
    NOTE: if Census is still shut down, compare any two hydrography data sources from different agencies. See Barbara's 10/10/13 Shutdown Workaround powerpoint for options.

If there is no hydrography in your area, see if you can expand your frame of reference enough to include a water feature. 

Optional data layers

The following data layers are good options for the other four layers but you can use others if you like:
Parks or other open space (city or MassGIS or compare the two)

  • a geocoded set of business locations (we'll do an exercise in class to demonstrate this)
  • open space / parks
  • schools
  • libraries
  • hospitals
  • land use or land cover (or compare the two)
  • parcels
  • building footprints
  • other transportation modes

Aerial photos/orthphoto cannot be one of your optional layers. You'll use these to assess the other layers.

What to post for your Assignment

In a short report, address the following questions, using a pretend project as a benchmark where appropriate. Use either local orthophoto imagery or the ArcGIS Data Online World Imagery data set as your "map of higher accuracy" against which to make comparisons - that's an assumption about the orthophoto accuracy which might not be true, but we'll use if for our purposes here. If metadata exists (e.g., in ArcCatalog or on an web site, like MassGIS or Tiger Documentation), use the metadata to help you understand the data set and the adequacy of the data for your proposed project (here's some instructions for viewing complete metadata in ArcCatalog - but remember, some data sets are not documented within ArcCatalog but may be documented elsewhere online or in a text file that comes with the data). Note: you don't have to do any formal maps for this assignment, but you should use graphic screen-prints to visualize the issues you're discussing (a picture is worth a thousand words!). We have a program called Snag-It on the lab computers (Start - All Programs - Desktop Publishing) or you can download an evaluation version from the Snag-It web site).

  1. Provide one-paragraph description of the project for which you are assessing the data and what the data needs to meet in terms of positional accuracy, required attributes, completeness, time period, and connectivity.  Tip: For estimating positional accuracy requirements, think about how far off the position could be and still work for the project needs.
  2. Briefly discuss the two different road centerline data sets in terms of their positional relation to the aerial imagery.  Include in your discussion:
    1. An estimate of the quantitative positional accuracy for each road centerline data set, compared to the aerial imagery
    2. Include some graphic examples to illustrate your points.
    3. Which data set would be best for your project in terms of its currency?
    4. Which data set would be best for your project in terms of positional accuracy?
    5. Which data set would be best for your project in terms of attribute information required?
    6. Which data set seems to you to be the most complete?
  3. Do the same as #2 above for the two hydrography layers
  4. Answer the following for each of your 4 optional data layers:
    1. Can you provide a quantitative assessment of positional accuracy for each of your optional data layers (e.g., +/- 20 feet)? Why or why not? (If you have metadata, you might base an answer on the source map's scale - here is the table of scales and the equivalent required accuracy from the US National Map Accuracy Standard for Paper Maps - just remember that when features are digitized from a paper map, additional errors are introduced! So digitized data is never better than its paper source unless it has been through some additional quality control process.
    2. Completeness: Is each optional data set complete to your knowledge? (Does it cover the area in question, are all relevant features that you need for your project?) How do you know that?
    3. Currency: Do you know the time period for each of the optional data layers? Is that adequate?
    4. Attribute accuracy: provide a qualitative assessment of attribute accuracy for critical attribute items (based on your project needs) in your optional data layers (e.g., land use codes, school names, open space management agency, etc). How adequate is the attribute information for your project needs?

Notes on where to locate the data sets

Road Centerlines:

  • MassGIS - under Infrastructure folder M:\State\MA\MassGIS\Infrastructure - EOTROADS_ARC; or online at MassGIS (the online data will be more up to date)
  • City street centerlines
    • City of Boston - under M:\City\Boston\DOIT\Vector\City_of_Boston_MA_Planimetric_GIS_Basemap geodatabase (look for the Road_cl layer) - Note: there is imagery in this same DOIT folder path.
    • City of Somerville - M:\City\Somerville\MIS (SomervilleStreets02.shp)
    • Or other cities on our M: drive
  • 2012 Tiger Roads - Census TIGER 2012 data set

Hydrography Data

  • MassGIS - under M:\State\MA\MassGIS\Physical_Resources - Hydro25k polygons
  • City hydrography - we have hydro/water data sets for Boston under M:\City\Boston\DOIT\Vector\City_of_Boston_MA_Planimetric_GIS_Basemap geodatabase (look for the Hydro_poly layer). For other cities, look under their folders.
  • 2012 Tiger Hydro - Census TIGER 2012 data set

Other data sets

For your optional data sets, if you're working in a place in Massachusetts, check out MassGIS or a city folder on our M: drive. Otherwise seach for your city or county or regional or state clearinghouse


  • You can always use the Imagery data set  - in ArcMap click on File - Add Data - Add Basemap 
  • Or check for imagery under our various city folders

Tip for reducing your data layers to just your area:

  1. Add all the data you will be comparing to an ArcMap session
  2. Make sure all layers you want to use for this assignment are turned ON.
  3. Zoom in to your area of interest 
  4. Click on the Select Tool
  5.  Drag a box over the area you want to study to select all the features there - it will look like a mess because every feature in the box in every layer is selected, but that's a good thing!
  6. One layer at a time, right-click on the data layer and choose Data - Export Data - this creates a new shape file you should name and save to your H: drive Assignment 4 folder or other personal folder)
  7. Add each to your data frame when prompted
  8. Remove the original layer after you create the subset
  9. Do this for each layer you will be studying, and eventually you will have just the subset layers.  

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