Lab9_instructions

1 Lab 9: Spatial Interpolation & Density Analysis _______________________________________ DUE: Thursday, November 10 th , 5:30 pm (15 points) Answer all the questions in a Word document named Lab9_YourFirstName_YourLastName.docx and upload it together with other deliverables to Canvas (Lab 9). It is not necessary to write out the questions. Use the task number and question number from the exercise instructions to identify your written answer. This exercise consists of one task covering the basic operations of spatial interpolation. You will be interpolating a precipitation surface for North Carolina using the Inverse Distance Weighted (IDW) method. Remember to SAVE your ArcGIS Pro project often so that you do not lose any (or as little as possible) of your work, should the software shut down unexpectedly. Data needed: • NCstation_sample: Contains a sample of 115 weather stations in North Carolina with data on long-term averages of annual precipitation (ANN_PRCP) based on 2006-2020 data from National Oceanic and Atmospheric Administration’s (NOAA) National Centers for Environmental Information (NCEI). • NC_boundary_prj: an North Carolina outline shapefile Both files are already projected in the same projection and hence there is no need to (re)project them. Task: Spatial interpolation using IDW You will create a precipitation grid using the IDW method. You can access the interpolation methods available in ArcGIS Pro by going to the Geostatistical Analyst toolbox (see image to the right). 1. In ArcGIS Pro, insert a new map frame and rename it Lab9task 2. Add NCstation_sample and NCbndry to the Contents pane

2 3. In the ribbon, click on the Analysis tab and in the Geoprocessing group, click on Environments • In the Environments dialog box, under Workspace , click on the folder next to the Current Workspace and set it to your Lab9 folder (just like you did for Lab8). Do the same for the Scratch Workspace. Your Lab9 folder is now the default input and output directory. • In the Environments dialog, under Processing Extent , click Same As layer: NC_boundary_prj o This tells the software, that when we do our interpolation, the extent of the raster should be limited to the NC boundary • Click OK 4. In the Geoprocessing pane, search for IDW and select IDW (Geostatistical Analyst Tools) 5. In the IDW dialog box, select the following: • Input features: NCstation_sample • Z value field: ANN_PRCP • Output geostatistical layer: IDW_geostats • Output raster: (leave blank) • Output cell size: 6000 o The output cell size units are based on the input layer’s units. If you go to the Properties of the NCstation_sample layer and the NC_boundary_prj layers, you will see that they are both in US feet. Hence, our cells will be 6000x6000 feet. • Power: 2 o The exponent of distance that controls the significance of surrounding points on the interpolated value. A higher power results in less influence from distant points. The default in ArcGIS Pro is set to 2 and given that your instructor does not have a theoretical justification for choosing another value, we will use the default. • Search neighborhood: o Defines which surrounding points will be used to control the output. Standard is the default. To read about the other options, please refer to the documentation . • Max neighbors: 15 • Min neighbors: 10 • Sector: 1 Sector

4 Q2: If we use this suggested power value instead of the default, will more distant points be given more or less influence over the estimated cell value? (1 point) __________________ 8. The graphic frame to the left in the Geostatistical Wizard shows station points and their weights (shown in color symbols) used in deriving the estimated value for a cell. • Click on different parts of the interpolated surface using the Explore tool (under Map in the ribbon) to see which control points were used in the prediction (/interpolating) of the selected cell’s value. • You can also check the weights that were used for each control point by clicking on the arrow next to Weights (15 neighbors). As you will see, larger weights are given to control points that are closer to the interpolated cell than those further away. 9. In the bottom right of the Geostatistical Wizard dialog box, click Next 10. The Cross validation portion of the Geostatistical Wizard allows you to draw scatter plots relating the predicted to the measured values at control point locations (Predicted tab), or the error against the measured values (in the Error tab). The error is the difference between the predicted and measured values. • Based on the fitted regression line in the Predicted tab, it appears that our predicted and measured values at control point locations are fairly close (this is a good thing) which some deviation (errors). • In the Summary tab on the right, you can see that our root-mean-square error which is 2.40 for this interpolation. 11. In the Geostatistical Wizard, click Finish • This gives you the Method Report which summarizes the parameters you have chosen for the interpolation. Click OK to close it.

5 You will now convert the IDW_geostats layer to a raster which you will call IDW_grid . 12. Right-click the IDW_geostats layer and click Export Layer → To Rasters 13. In the GA Layer To Rasters pane, enter 6000 (feet) for the Output cell size and specify IDW_grid for the Output raster (I’m getting a warning because I already have a layer called this – ignore) 14. Click Run 15. Clip IDW_grid by using NC_boundary_prj as the feature mask using the Extract by Mask tool (search for it in the Geogrocessing pane). Name the output raster IDW_precipitation and let the extraction area be Inside. Leave the rest of the options to their defaults. 16. Change the symbology of the IDW_precipitation by right-clicking the layer and click Symbology 17. In the Symbology pane, choose Classify from the drop-down and set the Method to Natural Breaks and 10 classes. Select a color scheme that goes from light to dark (of the same color). Such that it looks something like this:

7 20. Click Run. 21. Click on the IDW_contour layer and in the ribbon, go to Labeling 22. In the Label Class group, select Contour as your Field and click on the Label button in the Layer group The map now shows the labeled isolines. Map: Prepare a decent looking map showing IDW_precipitation and IDW_contour (include all map elements such as north arrow, legend, scale bar, etc.). Save your map as a .png file and insert the map in your Lab9_YourFirstName_YourLastName.docx. (7 points) Q8: What is the range of contour values in IDW_contour (hint: attribute table…) ? (1 point) ________________________________ Q9: If your instructor had downloaded a larger sample of weather station data (i.e., more control points) for you to use, how would this have affected the accuracy of the output surface in IDW_precipitation ? (1 point) ____________________________________________________

8 Rubric Task Description Points Map Map showing interpolated annual precipitation surface in graduated colors with 10 classes and labeled isolines. Map includes name, date, north arrow, scale bar, title, and legend. 7 9 questions Answered all questions completely and correctly. 8