handout

© Michael Alonzo
Active learning materials for Introductory GIS
Functionality comparison and needs matrix
Scenario: You are a consultant who is hired by a government agricultural agency to evaluate
precipitation data they’re interested in using. You know that the best way to analyze the dataset is using
GIS. They’re on board with this but you need to (a) convince them that they need something better than
Google Earth and (b) help them decide between ArcGIS and QGIS
Key steps:
1. Determine what the primary business needs are and what the accompanying GIS functionalities
will likely be
2. Compare the key functionalities in the three software packages
3. Based on your results, construct a needs matrix that will support your ultimate recommendation
Business needs:
1. Compute area of agricultural fields (polygon layer: XXX)
2. Evaluate the spatial clustering of precipitation gauge stations
3. Create accurate statistical surfaces for interpolated precipitation data
Questions you need to answer:
1. To what extent can these tasks be accomplished in Google Earth?
a. Area of polygons
i. Export ag_poly.shp to kmz in ArcGIS
ii. Open in Google Earth. (Q1) Can you calculate area of the polygon there? Poke
around the menus and search online for confirmation either way.
iii. Try this method:
1. Right click on your layer on the places menu (left) and Save Place As a
kml file (NOT kmz).
2. Navigate to this website: http://geo-news.net/index_geof.html
3. Run the area calculation on the ag_poly.kml
4. Also calculate area in ArcGIS in the Attribute Table using “Calculate
Geometry”.
5. (Q2) Are the areas the same or different? If they’re different why do
you think that might be?
6. (Q3) Which method for calculating area was more straightforward?
b. Spatial clustering of points (using something like the Nearest Neighbor Statistic)
i. Poke around the menus and search the web. (Q4) Is NNS functionality available
in GE?
c. Spatial interpolation: This is not available in Google Earth.
2. Comparing NNS functionality in QGIS and ArcGIS
a. QGIS:
i. Add the annual_precipitation.shp layer to QGIS.
ii. Use the menus or search the help menu to figure out how to run Nearest
Neighbor Analysis. Pay attention to how long it takes to run the algorithm.
iii. (Q5) Record the following outputs: Observed mean distance, Expected mean
distance, Nearest neighbor index.
b. ArcGIS
© Michael Alonzo
Active learning materials for Introductory GIS
i. Run NNS on same dataset. Access results here: GeoprocessingResults.
ii. (Q6) Which software package ran this analysis faster? Was the difference
significant?
iii. (Q7) How different are the results? Do you think QGIS and ArcGIS are
performing this calculation in the same way?
3. Comparing IDW interpolation in QGIS and ArcGIS
a. QGIS:
i. Run IDW on the precip attribute (RasterInterpolation). Make sure to click the
“Add” button for the appropriate vector layer and attribute. Change the
“Cellsize X” and “Cellsize Y” to 20 m. Choose output file and run.
b. ArcGIS:
i. Use “IDW (Spatial Analyst)” to run your analysis. Again, use 20 m cell size. Keep
other default settings.
c. Add your raster result from QGIS to ArcGIS. Use “Minus (Spatial Analyst)” to subtract the
QGIS output from the ArcGIS output. This will allow you to examine the discrepancies.
d. (Q8) Describe the range and spatial pattern of differences. Where are the differences
minimal? Where does QGIS overestimate with respect to Arc and vice versa?
e. Use the Info tool to click around in the far northwestern corner of the two images.
f. (Q9) In this area how do the values of the two outputs compare? Which output has
values that are closer to the measured values at the 4 rain gauges that are nearby?
Needs matrix.
3 = Very good (functionality aligns with needs, tool is straightforward yet flexible, computation is fast)
2 = Good (like “3” but slightly worse in one or several areas)
1 = Functionality available but there are significant drawbacks
0 = Functionality not available
GE
Poly Area
QGIS
ArcGIS
2
3
NNS
IDW
0
Cost ($)
Software recommendation: _________________________________________________________

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