Using the Density Processing Tool (Part 5)
Repairing Density Layers
Goals of the Lesson:
• Demonstrate how to repair inaccuracies and flaws in
density layers.
This portion of the lesson will use the Albany area.
To start:
• Continue with the same density layers and base maps
used in part 4.
Investigating the Accuracy of Density Layers
A review of Investigating the Accuracy of Density Layers:
In Part 4 it was shown that density layers may be inaccurate because:
1. Density layers are only as accurate as the underlying NAICS data
from which they are generated. If the data has mistakes in the number
of employees, the location, or the type of business, these will be
reflected in the density layer and will lead to misinterpretation of the
location and amount of density.
2. The underlying NAICS data may be out of date, leading to an
inaccurate layer.
3. Although a business may fall under one type of classification, it may
not have the expected relationship to the transportation network. For
example, some businesses classified as NAICS 42, transportation, may
entirely consist of office workers doing transportation related work.
Thus, the business will show high density, but not produce the expected
high levels of truck traffic.
Investigating the Accuracy of Density Layers
A review of Investigating the Accuracy of Density Layers:
4. Some businesses may perform multiple functions and thus may not
be classified as expected. For example, a business with a warehouse
may also sell parts at the same location, leading to its classification as
retail. The density layer will show the same thing: that there is retail
density at the location, but not warehousing density.
5. Activity location issues: the business may be listed in the database
under its mailing address, not the actual business location.
Investigating the Accuracy of Density Layers
A review of Investigating the Accuracy of Density Layers:
Solutions from Part 4:
In order to investigate the accuracy of a density layer it is suggested
that someone familiar with the area examine the density layers. In
general, the density layers should show density in the expected locations;
there should not be any surprises. If there are, those areas should be
investigated for accuracy.
Simple methods can be used to investigate the accuracy of a density
layer. First, a basemap can be used to see whether the type of
development is expected for the type of density. For example, if the
density shown is in transportation, one would expect to see an airport,
rail yard, or truck terminal. If the basemap instead shows a housing
development, it is likely the density layer is wrong. Google can also be
used to look at the businesses in the area in question and see if they
relate to the type of density shown by the layer.
Repairing Density Layers
Flaws in Density Layers can be Corrected!
Solutions:
Given that the typical user of the Density Processing Tool does not have
access to the underlying NAICS data used to generate the density layers,
it might seem that correcting flaws in density layers would be difficult.
However, problematic portions of density layers can be “cut out” and
removed by the user, providing a rough method of correction that does
not require the editing of the underlying NAICS data.
Repairing Density Layers
Repaired
DensityLayer.
Layer.
Flawed Density
Density layers can change dramatically when flaws are repaired!
Repairing Density Layers
Flaws in Density Layers can be Corrected!
Repairing density layers can lead to dramatically different results:
The previous slide showed the dramatic difference between a flawed
layers for the Albany area showing NAICS 11 (agriculture and forestry)
and a layer corrected by methods available to the typical user of the
Density Processing Tool.
It should be noted that whether a layer is flawed or not is to some
degree decided by the user. While in some cases businesses included
in the density layer may be blatantly misclassified in the underlying
NAICS data, in many cases a flawed layer may simply be one that does
not show what the user is looking for. In the previous slide, the flawed
layer included not only farms, but home run businesses and PO boxes
of farms located in urban areas, far away from farm operations. A layer
showing the density of farm and related agriculture operations was
needed, so the original layer was deemed flawed. Agriculture density
occurring in urban areas was removed, leaving a layer that showed a
more accurate picture of density in farming and agriculture operations.
Repairing Density Layers
Once again, the Flawed Density Layer.
Agriculture and Farming density in urban areas is all PO boxes
and offices; not the actual location of farm operations.
Repairing Density Layers
Repaired Density Layer.
With the agriculture businesses in urban areas removed, the rural
farming operations become clearly visible.
Repairing Density Layers
A Flawed Layer for Transportation (NAICS 48) Density
Now, for another example:
The area above represents office workers in transportation, so the
location would not create the expected truck traffic implied by its
density. Since the layer is being used to study truck generating
density, the area should be removed from the density layer.
Repairing Density Layers
Repaired Density Layer.
With the office density removed, the areas of transportation
density that represent actual transportation operations – and thus
are likely to generate truck traffic – are shown clearly.
Repairing Density Layers
Method 1: Using Another Layer to Improve a Density Layer
By investigating the NAICS 11 (Farming and Agriculture) layer in the
Albany area using the methods shown in Part 4, it can be shown
that much of the density occurs in urban locations and has nothing
to do with farm operations. In order to create a density layer that
shows density in farming and agriculture operations - which
generate truck traffic – the agriculture density in developed
areas must be removed from the density layer.
Step 1:
Investigate the density layer to determine where agriculture
density occurs in urban areas.
Repairing Density Layers
Using Farming in Albany as an Example
Repairing Density Layers
Using Farming in Albany as an Example
Agriculture and Farming density in urban areas; not the actual
location of farm operations.
Repairing Density Layers
Using Farming in Albany as an Example
Agriculture and Farming density in urban areas; not the actual
location of farm operations.
Repairing Density Layers
Using Farming in Albany as an Example
Based on the areal base map, all of these areas of density are
located in commercial or residential areas; they are not farms or
businesses related to farm operations that could generate truck or
rail traffic. Thus, they should be removed from the density layer.
Repairing Density Layers
Step 2:
Create a layer that can be used to extract the desired portions of
the density layer while leaving the flawed portions.
• In many cases the user will have to draw their own layer, but in
some cases (as in this example) using a readymade layer may make
sense.
Since the goal in this example is to remove density that occurs in
urbanized areas, a layer designating urbanized areas can be
downloaded from an online GIS clearinghouse.
In this case, a layer showing areas designated by the Federal
Highway Administration as being urbanized was downloaded from
the New York State GIS Clearinghouse.
Repairing Density Layers
This is how the urbanized areas layer appears.
Repairing Density Layers
It can be seen that most of the agriculture and farming density lies
within the designated urbanized areas. This density tends to mask
the lower levels of density that represent agriculture operations in
the rural areas. Thanks to the high density in urban areas,
most of the agriculture in the rural areas does not even show up on
the map.
Repairing Density Layers
The new layer to be used to extract the agriculture density from
rural parts of the Albany area can be created by subtracting the
urbanized areas layer from the existing Albany area layer.
Repairing Density Layers
Open the Arc Toolbox and go to:
Analysis Tools
Overlay
Erase
Repairing Density Layers
The Erase window will open:
• Set the Input Features to the layer designating the Albany area.
• Set the Erase Features to the urbanized areas layer.
• Give the resulting file a directory and name.
Repairing Density Layers
The resulting layer will be added to the table of contents in
ArcMap. This layer represents the part of the Albany area layer
that is not covered by the urbanized areas layer. Thus, it shows
the non-urbanized part of the Albany area.
Repairing Density Layers
Step 3:
Use the new layer to extract the agriculture
density from the rural part of the Albany area:
Go to Arc Toolbox:
• Spatial Analyst Tools
• Extraction
• Extract by Mask
Repairing Density Layers
The Erase window will open:
• Set the Input Raster to the agriculture density layer for Albany.
• Set the Input Feature to Mask Data to the newly created layer
showing the non-urbanized portions of the Albany area.
• Give the resulting file a directory and name.
Repairing Density Layers
Step 4:
The density layer for agriculture in non-urbanized portions of the
Albany area will be added to the map and the table of contents.
Adjust its display to match the other density layers and turn off
the original agriculture density layer.
Repairing Density Layers
The results are dramatic; with the urban agriculture density
removed, the top level of density drops, revealing the location
of density in the rural area.
Density for entire Albany area.
vs.
Density for the rural area.
Repairing Density Layers
The resulting density layer shows agriculture operations in rural
areas clearly.
Repairing Density Layers
Step 5:
Confirm that density in the new density layer represents
agriculture operations, using the investigation methods from part 4.
Repairing Density Layers
Step 5:
Confirm that the density in the new density layer represents
agriculture operations, using the investigation methods from part 4.
Based on these samples, it does appear that the new layer
represents agriculture operations.
Repairing Density Layers
Method 2: Improving a Density Layer by Cutting Areas Out
For the next example, flaws in the transportation density layer for
the Albany area will be corrected.
Repairing Density Layers
Using Transportation (NAICS 48) as an Example
Step 1: Locate flaws in the density layer.
It has been previously demonstrated that the transportation
density layer for the Albany area includes an area of density that
represents office workers, not operations workers. Only
operations are of interest, since they generate truck and rail traffic;
the offices should be removed from the layer.
Repairing Density Layers
Repairing Density Layers
Transportation density located in offices – not operations.
Repairing Density Layers
Step 2:
Create a layer that can be used to extract the desired portions of
the density layer while leaving the flawed portions.
In this case, the layer to extract the desired portions of density
from the flawed layer will be created manually.
Repairing Density Layers
Before proceeding, create a copy of the Albany area layer that can
be edited without destroying the original.
Right click on the layer in the
table of contents.
Go to Data
Go to Export Data
Repairing Density Layers
The Export Data Window will open:
Keep the default settings.
Set a directory and file name for the new layer.
Add the layer to the map.
Repairing Density Layers
Before proceeding, make the new layer the only selectable layer:
Right click on the layer in the table of contents.
Go to Selection.
Go to Make This the Only
Selectable Layer.
This will avoid accidentally
editing any other layers.
Repairing Density Layers
Next, start editing the layer:
Right click on the layer in the table of contents.
Go to Edit Features.
Go to Start Editing.
Repairing Density Layers
The Editor Toolbar will now become active:
Only two tools on the editor toolbar will be used
The Selection Tool…
And the Cut Polygon Tool.
Repairing Density Layers
First, use the Selection Tool to select the county that will be edited.
Repairing Density Layers
Next, use the Cut Polygon tool to cut out the area of offices, starting
from the edge of the county.
Repairing Density Layers
Now, select the cutout area and delete it using the delete key.
Repairing Density Layers
Now, select the cutout area and delete it using the delete key.
Repairing Density Layers
Finally, from the Editor button on the Editor Toolbar, select Save
Edits, and then Stop Editing.
Repairing Density Layers
Step 3:
Use the new layer to extract the transportation
density while leaving the offices behind:
Follow the same procedures used in the agriculture example, using
the transportation density layer as the input raster and the
newly created layer as the mask layer.
(see slide 26 for the agriculture example)
Repairing Density Layers
Step 4:
Adjust the symbology of the new density layer to match the others.
Repairing Density Layers
Repaired Density Layer.
Although hardly as dramatic as the farming layer example, the
office density is removed from the layer, clarifying where density
in transportation operations is located.
Repairing Density Layers
Step 5:
Confirm that density in the new density layer represents
transportation operations, using the investigation methods from
part 4.
Repairing Density Layers
Step 5:
Confirm that density in the new density layer represents
transportation operations, using the investigation methods from
part 4.
Repairing Density Layers
The remaining density appears to represent transportation
operations; both of the locations are rail yards. So, the revised
layer displays density in transportation operations while avoiding
showing density in office employment related to transportation.
Repairing Density Layers
Method 3: Improving a Density Layer by Segmentation
For the next example it will be shown how all density layers for
Westchester County can be improved by splitting them into
separate layers for the south and north.
Repairing Density Layers
Method 3: Improving a Density Layer by Segmentation
Up till this point, entire counties have been used to create density
layers for studying an area.
Step 1: Identify the Problems with the Density Layer:
In the case of
Westchester, the southern
and northern ends of the
county have different
characteristics, with the north
end being much more rural
than the density populated
south end that borders on
New York City.
Repairing Density Layers
Method 3: Improving a Density Layer by Segmentation
The difference in urbanization can be seen in areal maps:
= Intense Development
Repairing Density Layers
Method 3: Improving a Density Layer by Segmentation
This difference in urbanization is reflected in the various density
layers, with the south end of the county overwhelming the north
end.
= High Density
Repairing Density Layers
Method 3: Improving a Density Layer by Segmentation
Step 1: Identify the Problems with the Density Layer:
Due to the high density in the New York City suburbs of
Westchester County, studying density in the north end of the
county is very difficult; the low levels of density do not show up
well on the map.
= High Density
Repairing Density Layers
Method 3: Improving a Density Layer by Segmentation
Step 2:
Create a layer that can be
used to extract the desired
portions of the density
layer while leaving the
flawed portions:
In this case, it seems
beneficial to separate the
north end of Westchester
County from the south
end.
Repairing Density Layers
Method 3: Improving a Density Layer by Segmentation
The steps used to cut the Westchester County layer in two are
essentially the same as those used in Method 2: Cutting Areas Out.
The only difference is that the county is cut in two with the cut
polygon tool instead of having a small portion cut out.
Detailed instructions for Method 2 are shown starting with slide 37.
The General Steps are:
1. Export the data of the county.
2. Add the exported county to the map.
3. Make the exported county the only selectable layer.
4. Start editing the county.
5. With the editor toolbar, select the exported county.
6. With the editor toolbar, use the cut polygon tool to cut the
exported county layer along the divide between the urbanized
south portion and the rural northern portion.
Repairing Density Layers
Method 3: Improving a Density Layer by Segmentation
Detailed instructions for Method 2 are shown starting with slide 37.
The General Steps are:
1. Export the data of the county.
2. Add the exported county to the map.
3. Make the exported county the only selectable layer.
4. Start editing the county.
5. With the editor toolbar, select the exported county.
6. With the editor toolbar, use the cut polygon tool to cut the
exported county layer along the divide between the urbanized
south portion and the rural northern portion.
7. Select and delete the southern portion of the exported county.
8. Stop editing and save.
Repairing Density Layers
Method 3: Improving a Density Layer by Segmentation
The end result will be a
separate layer for the
north end of Westchester
(shown in purple).
Repairing Density Layers
Method 3: Improving a Density Layer by Segmentation
Step 3: Use the northern Westchester layer to extract the
Westchester density layers, creating new density layers for the
north end of Westchester.
Follow the steps used in the agriculture example, starting on slide
26.
Repairing Density Layers
Method 3: Improving a Density Layer by Segmentation
Step 4: Once the density layers for the north end of Westchester
are added to the map, adjust their display.
Repairing Density Layers
Method 3: Improving a Density Layer by Segmentation
The results are dramatic;
in this example the
location of wholesale
density in the north
becomes clearer.
Repairing Density Layers
Method 3: Improving a Density Layer by Segmentation
Step 5: Confirm that the new density layers appear to be accurate
using the methods previously introduced.
Repairing Density Layers
Conclusions:
Problems with density layers can be corrected without the need
to access the underlying NAICS data by editing the layers in ArcMap
and using them to extract the desired portions of density layers.
Methods to make layers used for extraction include:
• Using another geographic layer to edit a county layer.
• Cutting out and removing parts of a county layer.
• Segmenting a county layer.
Although several methods of correcting and improving density
layers have been introduced here, the user is also free to extract
density from any area by drawing their own layer in ArcMap.