Spatial Data Handling

A Very Simple Vector-Relational GIS Model

This page covers several techniques that a person could use to experiment with accessibility and exposute models using simple GIS functions and census data.

The basic workflow discussed here will be as follows:

  1. Propose to create, modify or remove some facility in a specific place,
  2. Consider the spatial mechanism that relates that facility to the surrounding population.
  3. Use the Draw tool to create a polygon representing this extent of the spatial effect that you prerdict wil carry the effect of your chasnge.
  4. Convert your graphic polygon to a shape-file so that it can be used in spatial queries.
  5. Use a Spatial Selection to select the census tabulationares (blocks, block groups or tracts) that are associated with your polygon.
  6. Use the Statistics tool to generate a quick summary of the number of people associted with the census tabulation areas that have been selected.

Steps 1, and 2, are covered in the page Describing Spatial Mechanisms in Decision-Making Context. Students will remember creating polygons with the draw tool form the tutorial on Nuts and Bolts of Cartography with ArcMap

Converting a Graphic Polygon to a Shape File

once you have outlined the footprint of your Accessibility or impacted area, you can convert it to a shape file. It is best to save this shape file in your project data folder so that you wil be sure to have it in your archive for future use.

References

Create a Shapefile representing the footprint of your spatial mechanism

  • Choose View > Data View or of you are in View > Layout View make sure you have the map window activated (See Focus the Data -Frame above) so that you are drawing in map space.
  • Draw a polygon that represents the area affected by your new facilty. Note: Use the polygon tool form the draw toolbar. If you try tocreate a polygon with a simple ring-shaped line, it wil not behave correctly when you use it to select other things that are inside of it.
  • Select the polygon and choose Convert Graphjics to Features from the Draw toolbar.
  • Save feature to a new data set of type Shapefile to a data folder within your project.

Selecting Features Based on Spatial Relationships

In our exploration of categorical data in a previous tutorial, we explored how SQL queries can be used to filter the rows of a table according to logical expressions such as Select Rows where the value of LandUse Is Equal To "Industrial". Or select rows where the value of Edit_Date is Greater Than "January 1, 2018". (to paraphrase SQL). Standard SQL offers a vocabulary of Comparison Operators which are used to order and create associations based on the different sorts of logis fir different tyopes of attributes. (character strings: categorical logic; dates: calendar logic; integers, and decimal numbers: artithmetic logic). The operators available in classic SQL are very powerful, and they are primarily involve one-dimensional relationships.

As we discuss in the post on Vector-Relational Database Management Systems, classic SQL has been extended with spatial data-types, like Points, Lines and Polygons and a whole new set of operators that order and associate rows in feature-attribute tables according to two-dimensional (spatial) logic. In ArcMap these spatial SQL operators are exposed through the Select by Location dialog. <.p>

References

Select Census Blocks that Overlap with your Spatial Footprint

  1. Choose Selection > Select By Location.
  2. Read the options in the Select by Location Dialog.
  3. For Target Features choose the census Blocks
  4. For Source Features, choose your hypothetical spatial impact or accessibility area.
  5. Here is a screen-capture of my Select by Location Dialog.
  6. Experiment with differnt spatial operators, and imagine the consequences of each of them in terms of probable over, or under-estimation of the blocks involved.

One of the powerful aspects of relational database models is that the output of one query/selection can become the input for another. As you can see in the Select by Location dialog, you have the option of selecting any feature within the target data-set or only selecting from the set of features that has already been selected (by some previous spatial or attribute query. These chais of selections can be quite powerful.

Calculating Summary Statistics for a Selection of Table Rows

Now lets say that you want to add up the sum pf population and households that was counted in those Census blocks that have ther centers within the hypothetical impact area. The simplest way to do this is to use the

References

  1. Open the Attribute Table for your census blocks.
  2. Verify that rows representing blocks associted with your impact area are selected.
  3. Right-Click the field heading Pop10 or Housing10.
  4. See that the Statistics Pop-Up tells you several things, including the number of blocks selected and the Sum of the values in the selected rows, fo rth field that you clicked.

Save Selected Blocks as a New Layer

Part of your problem in describing the summary numbers that you have found wil be to describe the relationship between the census areas that you have summarized with the hypothetical impact/accessibility area that you have defined. As you can see, some census blocks or block groups are not completely within the hypothetical zone. Depending on the density of these areas and the (unknown) distribution of the population within these areas the the estimate of number of 2010 residents that live within the zone wil be over estimated and under-estimated. To discuss this properly, you need to create a graphical hierarchy that illustrates these geographical relationships.

Part of the challenge is that the selection of block groups is delineated with the bright-blue lines -- which tends to dominate your graphical hierarchy. We ant to be able to control the way that the selected polygons are highlighted. Another challenge is that the selected set of blocks is volitile. If you select anything else, this set dissappears. The steps below discuss how to save the selected census polygons as a new layer which will persist and that you can symbolize to make them legible among other layers in your graphical hierarchy.

References

Save Selected Features as a New Layer

  1. Right-click your census layer that includes selected blocks or block groups and choose Selection > Create Layer From Selected Features
  2. A new Layer is created that includes just the selected features!
  3. Open the attribute table for this layer. Note that that it includes just the selected features.
  4. Rename this layer so that later on, you wil be able to figure out what it is. You might also choose a name that would be uesful in a legend.
  5. From the ArcMap toolbar choose Selection > Clear Selected Features. Now nothing is selected in your ArcMap project.
  6. Double-click the color patch in the layer's table of contents entry to access the symbology for the features. Important: if you try to adjust the symbology through layer peoperties, the layer will become unfiltered.
  7. Remove the fill and adjust the boundaries of the census polygons.