Exercise #8 — Descriptive Spatial
Statistics (Surfer)
Introduction to GIS Modeling, GEOG 3160,
University of Denver, Winter 2001
Name _____<enter your name>_____
Date _____<enter the date>______
From
the class website, download the data file Elev_pts_surfer.xls and place
it in
C:\Program Files\Golden Software\Surfer\Samples
Part 1 – Changing
IDW Interpolation Specifications
Access
the Surfer demo system by double-clicking on C:\Program
Files\Surfer\Surfer.exe.
1) Derive an IDW surface of the elevation data.
a)
Generate a map surface…
ü From the main menu click Grid
à Data, then select the Elev_pts_surfer.xls
file in the …\Samples subdirectory.
ü In the Grid Line Geometry
box, change the # of lines to 25 in both the X and Y directions.
ü Click on the folder icon in
the Output Grid box, specify Elev_IDW1.grd as the output file
name, then press Save.
ü In the Gridding Method
box, specify Inverse Distance to a Power as the gridding method.
ü Click OK to generate
an IDW surface using the default specifications.
b)
Create a wireframe plot…
From
the main menu click MAP à Wireframe à OPEN à OK to create a wireframe plot
of the Elev_IDW1 surface.
c)
Create a contour map…
From
the main menu click MAP à Contour à New Contour map à OPEN à OK to create a wireframe plot
of the Elev_IDW1 surface.
In
the Contour Map Properties dialog box, check the Fill Contours
box on the options tab. Click on the Levels
tab, click on the Fill button, and click on the Foreground Color
button to pop-up the Color Spectrum dialog box.
Click on the left arrow of the
spectrum and assign the color green; click on the right arrow and assign
red. Ctrl/click in the middle of the
spectrum and assign yellow. Click OK, OK and OK to
generate the color filled contour plot.
d)
Graphically overlay the contour and wireframe maps…
Shift/click
to select both
the contour and the wireframe plots.
Then on the main menu, click Map
à Overlay Maps.
Resize and position the
colorful plot in the top third of the worksheet.
2) Repeat the “generate a map surface…” steps
substituting Elev_IDW2 for the output file name.
Press
the Options button on the right of the Gridding Methods box.
Options dialog box for IDW gridding
method
Set
Power to 4, Ratio 2, Angle 45 and press OK, OK
to generate a surface using different gridding method specifications.
Repeat
the “create a wireframe plot…, create a contour plot…, and graphical overlay…”
steps and resize/position the plot to the middle third of the worksheet.
3) Generate a third surface and resize/position
its plot in the bottom third of the worksheet that has the specifications: ELEV_IDW3,
Power 4, Ratio 2, Angle 45, Search Factors 1,1,1 and 1 (see
below).
Search Tab for gridding
Question 1.
Screen-grab your work sheet with the three plots and embed below…
Briefly describe the effects of changing the gridding method
specifications. Your discussion should
include explanation of IDW algorithm and the mathematical impact of changing
the power, anisotropy and search factors.
Referring to the screen grab extend the discussion to how the
changes you introduced affected the interpolated surfaces.
Part 2 – Changing
Krig Interpolation Specifications
1) Create a new plot window by clicking on the New
button in the upper left corner of the main tool bar and selecting Plot
Document. Derive a Krig surface of
the elevation data as outlined below.
a)
Generate a map surface…
ü From Surfer’s main menu
click Grid à Data, then select
the Elev_pts_surfer.data file in the …\Samples subdirectory.
ü In the Grid Line Geometry
box, change the # of lines to 25 in both the X and Y directions.
ü Click on the folder icon in
the Output Grid box, specify Elev_Krig1.grd as the output file
name, then press Save.
ü In the Gridding Method
box, specify Kriging as the gridding method.
ü Click OK to generate
a Krig surface using the default specifications.
Click OK to the warning (any idea what it means?)
b)
Create a wireframe plot…
ü From the main menu click MAP
à Wireframe à OK to create a wireframe plot of the Elev_Krig1
surface.
ü Resize and position the
plot in the top third of the worksheet.
2) Repeat the “generate a map surface…” steps
substituting Elev_Krig2 for the output file name.
Press
the Options button on the right of the Gridding Methods box.
Options dialog box for Kriging
gridding method
Set
Kriging Type to Block, Drift Type to Quadratic and press OK,
OK to generate a surface using different gridding method specifications.
Repeat
the “create a wireframe plot…” steps for both of the new grids. Resize and position the interpolated
surface to the middle third of the worksheet.
3) Generate a difference surface between the
pair of Krig surfaces by Gridà Math and specifying
the Elev_Krig1.grd file as “Input Grid File A.” Specify Elev_Krig2.grd as input grid
file B. Specify Elev_%Krig1-2.grd
as output grid file C. Enter the grid
math function as C=( ( A – B ) / A ) * 100.
Plot,
resize and position the percent change surface (Elev_%Krig1-2.grd) to
the bottom third of the worksheet.
Question 2.
Screen-grab your work sheet with the three plots and embed below…
Make a general statement comparing the results of the IDW and
Kriging interpolations.
Which do you think is the “best” (what considerations lead to this
opinion)?
Briefly describe the effects of changing the Kriging gridding
method specifications. Your discussion
should include a general explanation of Kriging approach and the impact of
changing the type and drift options.
What is the data range of %change values?
Comment on the spatial pattern of the %change surface and suggest
any explanations for the differences.
Optional Question 8_1. Choose any two
interpolation methods (other than IDW or Krig) and generate interpolation
surfaces using the default specifications.
Generate %Change maps from the default IDW and Krig surfaces for both of
your grids. Prepare a brief report
describing the results and presenting the plots in an organized manner (use
SnagIt screen capture).
Note: five (5) extra credit points possible for each completed tutorial and write-up; optional assignments will be graded on their completeness, clarity and presentation.
Part 3 –
Descriptive Statistics
From
Surfer’s main menu click Open à Elev_Pts_Surfer.xls
(Excel file), OK.
Highlight
the data in column C (Value) then select Data à Statistics to pop-up the statistics
dialog box.
Statistics dialog box
Click
OK to generate a standard report.
Click Close once you have reviewed the report.
Select
Data à Statistics and click
the check boxes for the additional statistics noted in the following table.
|
|
Sample Data
|
Mapped Data (IDW Surface) |
|
Number
of values |
|
|
|
Sum |
|
|
|
Minimum |
|
|
|
Maximum |
|
|
|
Range |
|
|
|
Mean |
|
|
|
Median |
|
|
|
First
quartile |
|
|
|
Third
quartile |
|
|
|
Standard
error of mean |
|
|
|
95%
confidence interval |
|
|
|
99%
confidence interval |
|
|
|
Variance |
|
|
|
Average
deviation |
|
|
|
Standard
deviation |
|
|
|
Coefficient
of variation |
|
|
|
Coefficient
of Skewness |
|
|
|
Coefficient
of Kurtosis |
|
|
|
Kolmogorov-Smirnov
stat |
|
|
Complete
the “Sample Data” statistics column in the above table using the
information in extended report.
From
the current window, switch to the Plot 2 window by clicking Windowà Plot 2. Now click Fileà Open à Elev_IDW1 (Surfer file), Open. Inspect the grid values by clicking on the
small crosses superimposed on the contour map.
Close the data inspection window (click on the small “X” in the upper
right corner of the plot).
From
Surfer’s main menu click Grid à Convert à Elev_IDW1.grd, Open to open
that file (map surface) for converting to a different data type.
Specify
Elev_IDW1_xyz as the file name and ASCIIXYZ (.dat) as the file
type.
BE SURE to the specify
ASCIIXYZ as the file type before pressing the Save button.
Click
Open à Elev_IDW1_xyz (.dat
file), OK. This will open the
grid file as a Worksheet.
Highlight
the data in column C, select Data à Statistics, and generate the descriptive statistics needed to
complete the left column of the above table.
To get the definition of each of the
statistics, press Helpà Contents then Search. Choose “Statistics Results” as the topic,
then click on the “Statistics” link as shown in the figure. Review the definition of each of the
descriptive statistics you entered into the table.
Embed below the definitions for any of the statistics that might be unfamiliar to you…
Question 3.
Comment on the comparison of the statistics for the Sample Data and the
Mapped Data.
Would you expect the statistics to be identical, nearly the same,
loosely related, or unrelated for most interpolated surfaces? Justify your answer.
Would you expect the Elev_Krig1 mapped data’s statistics to be
more, less or unpredictable in its comparison to the sample data’s
statistics? Justify your answer.