13. Geoprocessing Spatial Data

You work for an environmental monitoring company conducting a project in South East Michigan. The project area straddles the boundaries of two adjacent counties, e.g., Wayne and Washtenaw.

As the GIS expert on the project, you are required to assemble several digital map layers pertaining to the geographic extent of the study area. Some of the map layers are available as separate files for Wayne and Washtenaw counties, so you will be required to merge and clip these files in order to create seamless layers for the study area. Other layers are only available for the entire State of Michigan, so you will have to clip these layers to the extent of your study area.

The shapefiles you are required to assemble are census blocks, soils, geology, roads, drinking wells and streams. You are also required to include a digital elevation model (DEM) of the area. You may include optionally include two orthophotos for extra credit.

Complete the task for your company using data from Michigan’s archived CGI.


13.1. Download Data

  1. Visit the Michigan CGI’s archived website at https://web.archive.org/web/20160201222536/http://www.mcgi.state.mi.us/mgdl/?action=thm

  2. Use the table below as a guide to what you should download. Since you are downloading the same type of data for both counties, in all you will download six of files.

Title

Layer Name

Notes

Geoprocessing Action

Census Blocks

Block Groups from MI Geographic Framework (v14a)

Merge and Clip

STATSGO Soils

Available for entire State

Clip

Roads

MI Geographic Framework Transportation (v14a)

Clip

Drinking Water Wells

Located under the Geology Section

Merge and Clip

Bedrock Geology

Located under the GEOLOGY Section.

Clip

DEM

Located under the TOPOGRAPHY Section

Clip

  1. Create a folder then unzip the files you downloaded to that folder. Place all of the unzipped files in the same folder. The illustration below shows some of my zipped and unzipped folders.

Downloaded Files

13.2. Create a new shapefile to represent your Study Area

  1. You are free to decide on the geographic extent of your study area. The only requirement is that it straddles the boundaries of the two counties.

  2. Open any pair of shapefiles for Wayne and Washtenaw Counties (e.g. the census block layers) and use them as a background for digitizing the boundaries of the study area. The shapefile type is polygon, so leave polygon as the Geometry Type.

Study Area

13.3. Merge Shapefiles

The pairs of shapefiles you downloaded that need merging should be merged before they are clipped. For example, the census block group files for Wayne and Washtenaw Counties need to be merged before clipping. In order to merge the shapefiles, you will use geoprocessing tools from Toolbox.

  1. Click on Vector | Data Management tools | Merge Vector Layers

Downloaded Files

  1. Fill out the dialog that appears. Under “Input layers”, enter the names of the two files to merge.

Downloaded Files

  1. Ensure that the path and name of your output are correct, then click Run. The resultant merged shapefile appears as shown below.

Downloaded Files

  1. Repeat the process for the other pairs of shapefiles you need to merge.


13.4. Clip All Shapefiles to the Extent of the Study Area

  1. Ensure that the Study area shapefile is loaded into QGIS.

  2. Click on Vector | Geoprocessing Tools | Clip.

Downloaded Files

  1. Fill out the dialog that appears. Note that the Input Features are the features you want to clip, i.e., the Merged Census Blocks. The Clip Feature is the file that serves as the cookie cutter, i.e., the study area polygon that you digitized.

  2. If necessary, re-specify a path and a name for the output. Click Ok

Downloaded Files

  1. The resultant file should appear as shown in the illustration.

Downloaded Files

  1. Repeat the process for the other shapefiles you want to clip.


13.5. Merging and Clipping the Digital Elevation Models (DEMs)

DEMs are floating point raster grids containing Z values that represent elevation. The steps for clipping rasters are different compared to those for clipping shapefiles. Below, I illustrate how DEMS are mosaiced (merged) and clipped in QGIS. Click on this link for more information: https://docs.qgis.org/3.4/en/docs/training_manual/processing/cutting_merging.html

Mosaicing DEMs

  1. Add the DEMs for Wayne and Washtenaw Counties to QGIS.

Downloaded Files

  1. Select the Raster menu, then Miscellaneous | Merge.

Downloaded Files

  1. Now click the ellipsis (three dots … ) next to the Input layers label to select the layers to merge.

Downloaded Files

  1. Fill out the dialog as shown below. Click Run to process the data.

Downloaded Files

  1. The results of the operation should appear as shown below:

Downloaded Files

Clipping the Mosaic DEM

  1. Add the study area shapefile to the project and drag it above the merged DEM.

Downloaded Files

  1. Select the Raster menu, then Miscellaneous | Clip Raster by Extent.

Downloaded Files

  1. Fill out the Dialog as shown below. Note that the output will be saved as a tif image. Click on Run.

Downloaded Files

  1. The results should appear as shown below.

Downloaded Files

Deliverables

The deliverables for this exercise are:

  1. One or two professional layouts showing the result of the merged and clipped shapefiles.

  2. A professional layout of the merged and clipped DEM.

The maps should be visually pleasing while at the same time reflecting a high degree of cartographic sensitivity.