Calculating historical agricultural land differences using QGIS - Revision history

EverettStCroix: /* Conclusion */

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Conclusion

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Step 6: Difference

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Step 5: Field Calculator

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Benchmark

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Conclusion

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Step 5: Field Calculator

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Step 4: Dissolve Polygons

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Step 4: Dissolve Polygons

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Benchmark

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Conclusion

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 ==Conclusion==
-When you are working with large datasets that have lots of polygons you can use the dissolve tool to make the data more manageable, however we need to be careful that we still keep the data accurate. If your computer is not powerful enough to do the calculations involved in this tool we also can use cloud computing to get this done in a more reasonable time. And in terms of the data we used in the example it turned out that using this method we were more easily able to tell that the amount of agricultural land declined by around 66.8% from 1983 to 2017 (not tested with fixed geometries but likely similar).
+When you are working with large datasets that have lots of polygons you can use the dissolve tool to make the data more manageable, however we need to be careful that we still keep the data accurate. If your computer is not powerful enough to do the calculations involved in this tool we also can use cloud computing to get this done in a more reasonable time. And in terms of the data we used in the example it turned out that using this method we were more easily able to tell that the amount of agricultural land shown in each dataset declined by around 66.8% from 1983 to 2017 (not tested with fixed geometries but likely similar).
 ==Limitations==

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 [[File:difference_grif.jpg|500px|thumb|center|Figure 14. Difference tool Settings]]
-Set the larger layer as the input, and the smaller layer as the overlay. In this case, ARI_1983_dis is the input, and ARI_2017_dis is the overlay. Ensure the file is saved in a known location.
+Set the larger layer as the input, and the smaller layer as the overlay. In this case, ARI_1983_dis is the input, and ARI_2017_dis is the overlay. Ensure the file is saved in a known location (Geopackage).
 ==Final Output==

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 ===Step 5: Field Calculator===
+Area should be measured in an equal-area projection. Reproject each dissolved layer with the Processing Toolbox tool "Reproject layer", setting the target CRS to "Canada Albers Equal Area Conic", and save the result. (This is quick because each dissolved layer contains only one feature.)
 Now that processing is complete, we need to calculate the area of each file. This will give us a number, that can then be converted into percent change.
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 <pre>% change = (A_2017 - A_1983) / A_1983 * 100</pre>
-A_1983 = ha, A_2017 = ha, % change =
 ===Step 6: Difference===

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 ARI 1983 feature count = 139000, Dissolve time = 1 minute 27 seconds || ARI 2017 feature count = 550000, Dissolve time = 1 hour 26 minutes 47 seconds
+Dissolve is a Valid tool to make large datasets easier to work with.
 ==Conclusion==

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 ==Conclusion==
-When you are working with large datasets that have lots of polygons you can use the dissolve tool to make the data more manageable, however we need to be careful that we still keep the data accurate. If your computer is not powerful enough to do the calculations involved in this tool we also can use cloud computing to get this done in a more reasonable time. And in terms of the data we used in the example it turned out that using this method we were more easily able to tell that the amount of agricultural land declined by 66.8% from 1983 to 2017.
+When you are working with large datasets that have lots of polygons you can use the dissolve tool to make the data more manageable, however we need to be careful that we still keep the data accurate. If your computer is not powerful enough to do the calculations involved in this tool we also can use cloud computing to get this done in a more reasonable time. And in terms of the data we used in the example it turned out that using this method we were more easily able to tell that the amount of agricultural land declined by around 66.8% from 1983 to 2017 (not tested with fixed geometries but likely similar).
 ==Limitations==

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 Now that processing is complete, we need to calculate the area of each file. This will give us a number, that can then be converted into percent change.
-When QGIS dissolves the ARI_2017_chk file, it keeps only one area entry. This means that area must be calculated from ARI_2017_chk, and manually added to the new dissolved layer (ARI_2017_dis).
+When QGIS dissolves the ARI_2017_Fixed file, it keeps only one area entry. This means that area must be calculated from ARI_2017_fixed, and manually added to the new dissolved layer (ARI_2017_dis).
 To access the Field Calculator, open the attribute table, toggle on editing and navigate over to the abacus icon. It is one to the right of the delete field icon highlighted in figure 3.
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-[[File:dis_fld_calc.jpg|500px|thumb|center|Figure 12. Update table value to include total are in hectares]]
+[[File:dis_fld_calc.jpg|500px|thumb|center|Figure 12. Update table value to include total area in hectares]]
 Above is an example of the updated value in ARI_1983_dis. By double clicking on the attribute, we can update it manually with the correct value.
+<pre>% change = (A_2017 - A_1983) / A_1983 * 100</pre>
+A_1983 = ha, A_2017 = ha, % change =
 ===Step 6: Difference===

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 Because the inputs had fixed geometries and the dissolve being run on newer QGIS software + more powerful computer with better RAM and CPU components the processing went very fast...
-In this test the dissolve took 1 minute 27 seconds for ARI_1983 (139000 features) and for the much larger dataset it took 1 hour 27 minutes] for ARI_2017 on the full fixed datasets (see the Benchmark section(s)).
+In this test the dissolve took 1 minute 27 seconds for ARI_1983 (139000 features) and it took 1 hour 26 minutes for ARI_2017 on the full fixed datasets (see the Benchmark section(s)).
 Dissolve is only suitable here because the question concerns total area. If you need to keep the attributes of individual polygons, do not dissolve.

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 ARI 1983 Geodatabase File Size = 413 MB, ARI 1983 Fixed Geometries File Size = 10.364 MB || ARI 2017 Geodatabase File Size = 200 MB, ARI 2017 Fixed Geometries File Size 198.864 MB
-ARI 1983 feature count = 139000, Dissolve time = 1:27 minutes || ARI 2017 feature count = 550000, Dissolve time =
+ARI 1983 feature count = 139000, Dissolve time = 1 minute 27 seconds || ARI 2017 feature count = 550000, Dissolve time = 1 hour 26 minutes 47 seconds
 ==Conclusion==

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 </td></tr></table>
+==Benchmark==
+Times and file sizes for [full data with fixed geometries] on [CPU: Intel i5 14400f, RAM: 16GB DDR5]
+ARI 1983 Geodatabase File Size = 413 MB, ARI 1983 Fixed Geometries File Size = 10.364 MB || ARI 2017 Geodatabase File Size = 200 MB, ARI 2017 Fixed Geometries File Size 198.864 MB
+ARI 1983 feature count = 139000, Dissolve time = 1:27 minutes || ARI 2017 feature count = 550000, Dissolve time =
 ==Conclusion==