Buffering in ArcMap tutorial

Transcription

1 Buffering in ArcMap tutorial 1) Open ArcMap 2) Bring in the Wards_2010 and Parks shape files from the city of Ottawa s open data portal. (NOTE: for the purposes of this tutorial, you could also bring in the play structures to see if any discarded syringes or needles are located within a short walking distance.) 3) Download the discarded syringe file. 4) Import the discarded syringe file into ArcMap.

2 5) Right-click on the discarded syringe layer and choose the Display XY Data option from the menu. 6) In order to ensure that the locations appear on the parks and ward layers, we will have to specify the coordinate system.

3 7) Go to the Edit button. 8) Choose the Select tab to find the appropriate coordinate system ( NOTE: Our textbook, Computer-Assisted Reporting, discuss

4 geographic coordinate systems on page 206 and 215). 9) Select the Geographic Coordinate Systems folder.

5 10) Choose the North America Folder, and NAD 1983.prj

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7 11) Select the Apply and OK tabs.

8 12) Select the OK tab. 13) Ignore the warning because ArcMap will create an Object-ID field when we save this layer as a shape file. So select the OK tab to place the points on the layers.

9 14) Now are layers are in line. We can to see how many of these discarded needles or syringes fall within, say, 50 metres of parks, which is roughly walking distance for moms or dads with small children. As we noted above, you could also pull in play structures. But let s stick with parks for the purposes of this tutorial. 15) Right click on the syringe layer and save it as a shape file. 16) Select the Syringe layer, and then go to the Selection tab at the menu across the top to obtain the Select By Location option. 17) Select Parks. Our Source layer is the syringe file. And let s choose a 50 metre search distance or buffer and be sure to specify that discarded items are to be found within a distance of

10 the source layer feature. Your dialog box should look like this: 18) Select the Apply tab. And Open the layer s attribute table to see how many discarded items were located within 50 metres

11 of parks. 19) Close the attribute table. 20) Right click on the layer and select Export Data, which will produce a dialog box that allows you to export the selected features. (NOTE: In Qgis, you have to specify that you want to export the selected features once you get the export dialog box. )

12 21) Name the shape file with a recognizable label, something like Syringes50MfromParks, and save the file in the same folder as the other layers. 22) Once the new layer is added to the map, turn off the layer with the full complement of discarded items. 23) So far, we ve done enough analysis to produce the kinds of stories the Toronto Star and CBC have done. You can also export this layer as a csv file. 24) Now that we know the number of discarded syringes close to parks, let s see how many of these items are located in particular wards. 25) To do this, we will count the number of discarded items in wards, by joining the two layers. (IN Qgis, this is function is called Points in Polygons. )

13 26) Right-click on the wards layer, and select Joins. 27) Since we re doing a spatial join, using the geographic coordinates from the two files, we will change the option under the What do you want to join to this layer section at the top of the dialog box. 28) The layer we want to join is the one selected, the Syringes50MfromParks.

14 29) And we want to SUM the number of discarded syringes within each ward. 30) Let s save the new layer in the same folder with the other files, making sure to give it a name that clearly distinguishes it from the other layers.

15 31) Let s call it Syringes50MfromParksInWards. 32) The new layer is one colour. But if we open the attribute table, we can see that ArcMap has counted the discarded syringes nearest parks in each ward.

16 33) Close the attribute table, right click on the new layer to obtain the Properties dialog box. 34) Select Symbology, Quantities, Count in Values, and select a range. 35) Let s stick with the five classes and this colour ramp.

17 36) Select the Apply and OK tabs.

18 37) Return to the Properties dialog box to select the ward labels. 38) Select the Apply and OK tabs.

19 39)