In my previous post I gave an overview of how to create a shapefile from scratch, where we created a point layer to identify places and neighborhoods in NYC. In this post, I’ll pick up where we left off.
Whenever you create new features in a shapefile, ArcGIS automatically adds a couple of fields, including an auto-number ID field that uniquely identifies each feature. This was sufficient for our example as the 291 place names we were working with do not have a standard ID number that represents them. If we were creating features that did have a recognized ID number or code, we certainly would want to add an additional field to hold that number. This would allow us to share and relate our data to other datasets that use that conventional ID. For example, if we had a layer with the 50 states, we would want to have a FIPS number or the two digit postal code for each state in the attribute table, so we could relate our states feature to the zillions of other state-based data tables out there that also use these codes.
It’s also helpful to add other identifiers to relate our place names to some larger geographic area. Why? Let’s say we want to filter our neighborhoods by borough – perhaps we just want to label neighborhoods in Manhattan or calculate distances only between places that appear in the Bronx. It would be useful to have a borough code or some other code associated with each of our place names for running queries.
As it turns out, the City of New York does use a standardized system of three digit codes to identify all boroughs and community districts in the city. In our example, the code for Manhattan Community District 12, which contains Inwood and Washington Heights, is 112. The first digit identifies the borugh and the second two digits identify the district. It would be a good idea to assign each of our neighborhoods this district code, so we could filter our features by either borough or district.
When we create each feature, we could manually type in the code in it’s own field just like we added the neighborhood names, but that would be rather tedious – and unnecessary. A better choice would be to do a spatial join. Whereas a “regular” join allows us to join attribute tables based on a common ID field, a spatial join allows us to assign attributes to one layer based on their geographical relationship to another layer.
In the Table of Contents, right click on the neighborhoods layer and choose Joins and Relates – Joins. We’ll get the familiar Join dialog box. However, if you hit the first drop down box that says Join Attributes From a Table and choose Join Data Based on Spatial Location, we’ll get the options for doing a spatial join. Choose the community districts as the layer to join to the neighborhoods, and since we’re joining points to polygons we’ll choose parameters that are relevant for relating these two features. In this case, give each point (neighborhood / place) the attributes of the polygons (districts) that it falls inside. ArcGIS will create a new point layer with the joined fields when you hit OK. Open the attribute table of the new point layer, and you’ll see the additional fields, including the community district numbers. You’ll also get some rather useless fields from the district layer, like the length and area of each district, which you can safely delete.
So instead of tediously entering these numbers by hand for each neighborhood, we simply run the spatial join process once (after we’ve finished adding the points for all 291 neighborhoods) and the IDs are automatically added.