Exercise 3: Distance and Overlay
Assigned: Oct. 15/16
Due: Oct. 22/23
Lectures
Lab Schedule
Lab Syllabus
Deliverable: Answer all questions and submit them as described in: Exercise Deliverables
Objectives:
 Extract data with polygon boundaries
 Zonal Statistics overlay with polygons on raster
 Perform and understand the results from table joins
 Calculate distances with the raster data structure
 Overlay raster data with raster calculator
 Use raster calculator conditional operation to selectively assign cell values
Introduction:
This lab exercise is concerned with the distance and overlay operations for GIS. These are
fundamental origins of spatial analysis. Ian McHarg’s Design with Nature is often regarded as
the first document to popularize the overlay approach. However, academic and practical work to
computerize overlay predate McHarg and are attributed to Dr. John Snow (1854) Roger
Tomlinson (CLI) and The Harvard Lab for Computer Graphics and Spatial Analysis (SYMAP)
and the University of Washington Department of Geography. (see:
http://isites.harvard.edu/fs/docs/icb.topic39008.files/History_LCG.pdf)
In this lab exercise you will compute distances with the raster data structure as the main learning
objective. After that, you will use overlay to analyze distance relationships in specific zones to
better understand relationships contributing to a “vulnerability perspective” about protection of
coastal habitats. But first, let’s consider why we would want to use distance and overlay.
Stream buffers:
Buffer zones are an important consideration to preserve the health of aquatic habitats.
Washington State has laws mainly oriented for the forest industry, but there are other concerns
for activity in buffer zones beyond this issue.
See at least one of the links below for context about buffers in drainage areas:
Design of Forest Riparian Buffer Strips –
See section titled “Washington: Riparian Management Zones (RMZs)”.
Chapter 222-24 WAC ROAD CONSTRUCTION AND MAINTENANCE
See page 24-7 on “Water crossing structures”.
General insight about why stream buffers matter:
In a buffer zone every tree has a purpose (see this one for buffer cross section)
Buffer Zones Preserve Washington's Cool, Clean Water (as seen on TV)
Overlay of impervious surfaces and drainage areas:
An impervious surface does not allow water to penetrate from a surface water location to a
groundwater location. Impervious surfaces promote substantial water runoff. The area of
impervious surface within a drainage area has been shown to have a direct relationship to the
health of aquatic habitats. Drainage areas are often thought of as the “land area” that drains into a
water body, however, even water bodies drain to other water bodies – like land drainage to
rivers, and river drainage into Puget Sound. Drainage areas come in many sizes. Drainage basins
are very large, e.g. like Puget Sound Georgia Basin. A drainage basin is composed on many
“watersheds”. A watershed is composed of subwatersheds. The U of Washington is within the
Ravenna subwatershed (Ravenna Creek drains to Lake Washington) that is part of the Cedar
River watershed.
See at least one of the links below for context about impervious surfaces and drainage areas:
Forest Cover, Impervious Surface Area, and Urbanization Impacts in King County
Read introduction and Fig. 1
Impervious Surface Mapping in Coastal New Hampshire (2005)
See this for example on use of roads and landcover (p.4 to 8)
Impervious Surface Mapping and Change Monitoring Using Landsat
Lots of cool color graphics
Roads and parking lots, and building footprints are the biggest contributors to impervious
surface. Roads are by far the biggest contributor. It is possible to use the length of roads within
an area as an approximation of the impervious area. What information is missing from the proxy
if we were to directly compute the area of roads?
Part A: Zonal statistics overlay of bathymetry in bird habitat
Help topic: “Zonal analysis by attribute.”
[Question 1]:Which levels of measurement is/are suitable for use in zone statistics?
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Load R:\GEOG460\data\PugetSound\UW_Oceanography\psdem\psdem
Check data frame properties for the default coordinate system setting.
This next data set is stored in as ESRI personal geodatabase (.gdb). For the moment treat
the gdb file the same as a folder. The difference between a folder and a gdb is the gdb
has built in environment settings that control the layers and their relationships.
Load: R:\GEOG460\data\PugetSound\NOAA\esi\1262_pugetGDB\PugetSound.mdb\
birds_polygon
Check the coordinate system of birds_polygon
[Question 2]: Which of these coordinate systems is suitable for what kinds of distance
operations? Set your analysis environments accordingly.
[Question 3]: This lab applies psdem in three spatial dimensions. What is the cell size and
measurement unit for horizontal x,y space. ‘Z value’ is a term used to describe measurement in
three dimensions. What is the measurement unit and reference system for psdem’s z value.
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The data needed for this analysis is located in the gdb in a table named biofile.dbf.
Both bird_polygon and biofile share a key field named ‘rarnum’ to link them together
with a join.
General information on the esi database is at:
o R:\GEOG460\data\PugetSound\NOAA\esi\ ESI_readme.pdf
o R:\GEOG460\data\PugetSound\NOAA\esi\ esi_mapping.pdf
The metadata file is:
o R:\GEOG460\data\PugetSound\NOAA\esi\1262_pugetGDB\
1262_pugetGDB_metadata.pdf.
o Also useful is information assembled by the TA’s in
R:\GEOG460\data\PugetSound\NOAA\esi\1262_pugetGDB\ESI Data join
notes.doc
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Rightmouse click on bird_poly –>Join and Relates –> Join.
In the dialog choose these settings:
o Join attributes from a table (that is, join attribute fields)
o Field in the layer: RARNUM
o Browse for a join table and select biofile
o Field in the table: RARNUM
Find the field(s) for RARNUM in the attribute table for bird_polygon.
[Question 4]: The two fields named RARNUM (each specific field) are each used in different
ways. How?
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Take note of the fields birds_polygon.BIRD_ and OBJECTID.
Close the table and execute this SPAN command
Spatial Analyst toolbar -> Zonal Statistics
o Use these settings for the dialog:
o Zonal dataset: bird_poly
o Zone field: birds_polygon.BIRDS_
o Value raster: psdem
o Ignore NoData in calculations: checked
o Join output table to zone layer: checked
o Chart statistic: unchecked
o Direct the output file to the working directory
This operation may take several minutes depending on server and network traffic; be
patient.
A summary table of the operation is displayed when the calculation is complete. Dismiss the
summary table and open the attribute table for birds_polygon. The Zonal Statistics table with
values calculated from psdem has been joined to this table so results for each polygon are
available for analysis and map display.
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Open the attribute table for birds_polygon and scroll to the right showing the fields where
the summarize zones table has been joined
[Question 5]: What is the meaning of the values in the fields: zstat#.max, zstat#.min,
zstat#.mean? Why do many records have null values?
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Map the species type using biofile.NAME.
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Use symbology on the fields biofile.NAME (which is the common name of the species),
zstat#.max, zstat#.min, zstat#.mean to visualize the result.
Open the birds_polygon attribute table and do the following.
Right click on the field name biofile.NAME and select summary. A dialogue will appear.
Scroll to the bottom of the field options and open the choice for the fields: a) MIN and
check average, b) MAX and check average, and c) MEAN and check average. Name the
output file accordingly.
When complete, add the table to the map; it is added to the TOC. Right mouse click to
open the table. Right-mouse click on the field names in a table if you want to sort.
[Question 6]: Which three bird species have their habitat in deep water? Which three bird
species have habitats on the shoreline?
Part B: Vulnerability of streams to hazards from nearby roads
The following links provide background information about hydrologic units (HUs) and
hydrologic unit codes (HUCs).
Hydrologic unit (HU) maps at the USGS
http://www.ecy.wa.gov/programs/wq/funding/2009/HUC-StreamreachCodeInstr.pdf
(the correct link for the URLs in the page above is: http://nhd.usgs.gov)
Example application: Urbanization and Forest Change Technical Background Document
Metadata: Hydrologic Unit Boundaries for Oregon, Washington, and California
[Question 7]: What is a HU and what is a HUC? How is a HUC used?
Feature extraction with vector overlay:
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Load R:\GEOG460\data\PugetSound\WaHydroFrame\HUC_6th
Use Toolbox -> AnalysisTools -> Extract -> Select to extract the HUs in the
Stillaguamish subbasin using the field “SUBBASIN_N”. The ‘SQL’ query builder works
just like the dialog for Select by Attributes’. Direct the output to your working directory
and name it ‘stilla_hu’.
Load the PNW River Reach data
R:\GEOG460\data\PugetSound\PNWRF\PNWRF3_2001.mdb\Hydrort_PNW_100k_lcc
Use “intersect” to confine your analysis of rivers and roads to the Stillaguamish Basin. In
other words, we want to exclude the rivers and roads that are not in the Stillaguamish.
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Use Toolbox -> Analysis Tools -> Overlay -> Intersect. Enter stilla_hu and
Hydrort_PNW_100k_lcc as the input features and direct the output to your working
directory. Name the output file ‘stilla_riv’.
Load the 1997 version of GDT Dynamap roads:
R:\GEOG460\data\PugetSound\transportation\GDT_1997\gdt_1997
Create a subset of gdt_1997 from the Stillaguamish using ‘Intersect’ the same way you
created ‘stilla_riv’. Name this output stilla_rd
[Question 8]: What occurred in the spatial intersect operation involving stilla_hu and gdt_1997
that produced still_rd?
[Question 9]: Why are there more fields in stilla_rd than gdt_1997?
Raster ‘Euclidean distance’ operation on roads and rivers:
Before performing the Euclidean Distance operations, we must set the analysis environment
coordinate system settings both in the main program and in SPAN:
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Tools -> Options -> Environment Button -> General Settings -> Output Coordinate
System -> select: ‘same as stilla_hu’
Now set the SPAN analysis options so the output is the same as the data frame.
Finally, set the data frame coordinate system to be the coordinate system for stilla_hu
(NAD_1927_Lambert_Conformal_Conic).
Now we are ready to begin the Euclidean Distance operations:
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Apply Toolbox -> Spatial Analyst Tools -> Distance -> Euclidean Distance with the
following parameters:
o Input raster or feature: stilla_rd
o In your working directory name the output: rd_dist25
o Output cell size: 25
o Leave the other input options blank.
After clicking OK the process will take a minute to complete.
When done display the output and evaluate the cell values nearest and furthest from the
road vectors.
Repeat the same Euclidean Distance operation for stilla_riv with the same parameters
used above. Name the output riv_dist.
[Question 10]: What’s the measurement unit and measurement framework for cell values in
riv_dist25 and rd_dist25?
Raster overlay on distance grids
Rd_dist and riv_dist are the product of distance functions on raster data structures. This next
section uses these measurements in overlay operations to evaluate distance relationships between
roads and rivers.
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Use raster calculator to do the following:
o Add riv_dist25 to rd_dist25. It will be added to the TOC as “calculation”. Export
this layer to your folder and name it add_rivrd
o Subtract riv_dist25 from rd_dist25. It will be added to the TOC as “calculation2”.
Export this layer to your folder and name the output sub_rivrd
o Divide sub_rivrd by add_rivrd. Export this layer to your folder and name it
div_subadd
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Examine the cell values in the output grids to learn the positions of the extreme values,
cell with value of zero, cells with NoData and those closest to rivers and roads. Use
symbology to help visualize the distribution of values across the basin.
[Question 11]: Fill in the table below with the meaning of cells with these values found in each
of the distance calculations performed above. You may enlarge the table and write answers in
directly or present answers in list form using the row/column index numbers together.
Examples:
3i) No cells with this value.
1h) A road cell 25 units from the closest river cell.
Data set
a)
1000
b)
-25
Cell value
c)
d)
-1
0.5
e)
0
f)
0.5
g)
1
h)
25
i)
1000
1) sub-rivrd
2) add_rivrd
3) div_subadd
Overlay raster distance calculations to evaluate HU vulnerability
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Use Toolbox -> Spatial Analyst Tools -> Conditional -> Con
o Input conditional: riv_dist
o ‘True’ raster: div_subadd
o ‘False’ raster: blank/nothing
o Output raster: working directory\riv_con
o Value <= 300
Adjust the symbology for riv_con to use a color ramp with three colors. Take note of the
color for low and high cell values.
Since roads that are adjacent or cross rivers can have a negative impact on aquatic health it is
worthwhile to identify which HUs are most vulnerable and deserves further assessment.
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Use Zonal Statistics to account for the values of div_subadd in each HU.
Spatial Analyst toolbar -> Zonal Statistics
o Zone data: stilla_hu
o Zone field: SUBWAT_NAM
o Ignore NoData: checked
o Join output table: checked
o Chart statistic: mean
o Output: working directory\znst_con.dbf
Consider the chart drawn from the analysis. Draw stilla_hu with a symbology like this:
o Quantiles: Graduated colors.
o Fields value: znst_con.MEAN
o Color ramp: use a bi or tri color ramp
[Question 12]: Which HU seems most vulnerable given the values from the summarize zones?
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Create a new field named ‘my_area’ and another named ‘nrmlize’ to represent
vulnerability of a HU in terms of potential impacts of roads on streams.
Use ‘calculate field’ on my_area to calculate the area of each HU.
For nrmlize, divide the value of znst_con.MEAN by stilla_huc.my_area and multiply it
by 1,000,000,000. Division of a very small number with a very large one is difficult to
interpret with the limitation of number fields in a table.
Sort the table using the nrmlize field.
[Question 13]: Now which HU could be considered most vulnerable based on nrmlize? What
was the purpose of normalizing the HU scores? Did the division of div_subadd by the area of the
HU contribute to the evaluation of vulnerability?