Spatial Data Bootcamp
Buffer Exercise
Data Collection

Pre-requisites:

  • Setup
  • Basics

Objecties:

  1. Perform simple spatial function on vector data
  2. Further understand projections

Exercise: Why we should all check our data

Why we should all check our data beforehand

Inspecting our data is before we attempt to perform any calculation, analysis, or spatial function is very important. To prove this, we are going to perform a simple buffer analysis. Spatial analyses are explained in Spatial Analysis.

  1. Open QGIS and import faults_100k.shp through iRods:
       /iplant/home/shared/aegis/Spatial-bootcamp/data-collection/faults_100k.shp

    Or download, unzip, and Add Vector Layer Spatial Data Bootcamp: Vector Analysis - add vector data:
    fault_100k.zip
    Spatial Data Bootcamp: Vector Analysis - streams and faults
  2. Buffer fault lines to 100m:
    • Menu Bar > Vector > Geoprocessing Tools > Buffer(s)
    • Configure inputs as follows:
      Input vector layer: fault_100k
      Buffer distance: 100
      Output shapefile: fault_buffer_100m.shp
      Add results to canvas: (Checked)
      Click OK to perform analysis, Close to exit the tool
      Spatial Data Bootcamp: Vector Analysis - buffer analysis
    • Zoom in to a buffer, close enough to see the width

      Spatial Data Bootcamp: QGIS - buffered faults
    • Open the meaure tool Spatial Data Bootcamp: Vector Analysis - measure tool
    • Measure the width of the buffer. Notice how the total width is only about 62 meters, or 200 feet (buffers both sides of line; 100 feet X 2 = 200 feet). Current map units are determined by the CRS, in this case EPSG:2927 is in US feet and we need meters. Simply changing the CRS in Project Properties does not give us the correct results- this is still measured from EPSG:2927 US feet.
      Spatial Data Bootcamp: Vector Analysis - measure buffer

      Important:
      There are two methods to perfoming a buffer analysis on our shapefile. The problem here is that our CRS is EPSG:2927 (in US feet) and the buffer distance is based on map units (still US feet). The two methods are: convert 100 meters to US feet and use this value in the buffer analysis while maintaining EPSG:2927 (in US feet); OR, reproject the layer thus creating a new file with a CRS in meters, and then reprojecting back to EPSG:2927. We’ll go with the former.

  3. Now we need to perform the buffer analysis with the correct method. Remove the incorrect buffer layer: fault_buffer_100m.shp
  4. Convert 100 meters to feet… = 328.084 feet.
  5. Again, open the buffer tool: Menu Bar > Vector > Geoprocessing Tools > Buffer(s)
  6. Buffer fault_100k with correct distance:
    • Configure inputs as follow:
      Input vector layer: fault_100k
      Buffer distance: 328
      Output shapefile: fault_buffer_100m.shp
      Add result to canvas: (Checked)
      Click OK to run (overwrite necessary), and CLOSE to exit tool

      Reminder:
      Buffer distance = 100 meters * 3.28084 feet = 328.084 feet
      1 meter = 3.28084 feet

  7. Confirm 100 meter buffer on faults:
    • Open measure tool Spatial Data Bootcamp: Vector Analysis - measure tool and confirm 200 meter total width (100 meters * 2 = 200 meters)
      Spatial Data Bootcamp: Vector Analysis - measure faults buffer

You’ve now tackled a large issue related to map units and projections. Not necessarily the buffer, but the consequences of not knowing understanding your data before working with it.

It’s always good practice to check the metadata before working with it.

You do not need to save this project or the data.

    Lessons

  1. Setup
  2. Basics
  3. Data Collection
    1. Metadata
    2. Data Sources
    3. Buffer Exercise
  4. Data Hygiene
  5. Spatial Analysis
  6. Cartography