CS 128/ES 228 - Introduction to Geographic Information Systems

Lab 4: Raster Formats 

Goals:

        By the conclusion of this lab period, you will have:

  1. Learned the difference between raster and vector data formats as used in GIS.
  2. Learned how to add raster data to a GIS.
  3. Explored some of the limits of raster data.
    4.  

What Is Raster Data? 

  1. A close look at a raster data layer.  Start ArcMap on one of the lab computers and open the ArcMap file from last week’s lab. Zoom to the intersection of Constitution Avenue and Route 417.  Describe in considerable detail how you could have found this intersection if you were totally unfamiliar with the area.  How well does the aerial photograph image of the intersection align with the digitized road layer from the U. S. G. S. 7 ½ minute topographic map? [Hint]


  2. What’s a pixel?  Zoom in to a scale of exactly 1:200; there is an easy way to do this.  There is also a hint, if you need it. center on the aerial image of the intersection.  Describe what you see.  How does the image depict the road surface?  What is the spatial resolution of the image, in feet? [Hint]  Repeat at a smaller and a larger scale (report the scales you pick, and whether each is smaller or larger than 1:200).  How does a scale change affect the cell or pixel size of the aerial photograph?  


  3. Comparing raster and vector data.  Zoom out a bit and pan to the State Highway version of Rt. 417.  How, other than location, does it differ from the aerial image?


Using Raster Data In A GIS

  1. Rasterizing vector features.  Download the Word file “6x10grid.”  Examine the (fictitious) map and identify the features symbolized.  Use the colored blocks at the top of the map to “rasterize” the features. [Hint]  Spend at most 10-15 minutes on this task – make sure you rasterize at least part of every feature, but you needn’t complete the rasterization if it seems to be taking too much time.  Add your names to the document below the newly rasterized picture.  Save as “6x10grid – your names”.  Repeat for the 12x20grid, recognizing that this will take a bit longer.
    2.   How did you choose what color to shade a cell?  What are the advantages and disadvantages of the 12x20 grid as opposed to the 6x10 grid?

  2. A map of a different (sized) pixel.  The NYS GIS Clearinghouse web site also has a New York State Land Use map.  In order to save time downloading and unzipping this file (which is huge, because it covers the entire state), we’ve downloaded the file, ny_nlcd.tif, already.  Add the Land Use map to the Olean Quadrangle data frame.  Turn off the aerial photograph display and use the approach in step 2 to measure the pixel size of the Land Use map.  Pan to the Allegheny River near campus and compare the land use layer’s depiction of the river to that of the aerial photographs.


  3. Limits of the raster format.  Discuss the effects of cell size on raster representations of spatial features:  How do the size and orientation of the feature affect the result?  What are some problems that arise?  What cell size (or resolution) would be needed to adequately represent all the features on the map?  How many cells would the resulting grid contain?

 

 

To Hand In

You are to create a text document including the answers to the questions posed in this lab, a copy of the map showing all the aerial photographs you added in the lab.  Attach a cover page and email the entire document to your instructors.  You may send the two rasterized maps as individual files since it is easy to screw up the alignment of the table and the map as one copies and pastes.

 

Help Policy

       Help Policy in Effect for This Assignment: Group Project With Limited Collaboration

       In particular, you may discuss the assignment and concepts related to the assignment with the following persons, in addition to an instructor in this course: any GIS  instructor and any student enrolled in CS 128/ES 228.

       You may use the following materials produced by other students: materials produced by member of your own group.