Adding and populating fields with model builder
Problem: You need to consolidate datasets based on theme, region, or some other criteria
This is a comment data management task. You may receive layers that are unnecessarily split into a lot of small files. For instance, I often see roads divided into separate layers for each road type. Highway, major, minor, trail, etc. — all in their own shapefile. It is useful to be able to distinguish between road types on a map, but it is better done through the use of symbols or definition queries than by having an individual layer for each.
By the same token, themes are often split up by region. You might have all the road types together, but in a separate shapefile for each county, state or country. You might rather have a single worldwide or countrywide coverage. But as with the road types, it is a good idea to maintain the ability to distinguish between region, in case the need should arise.
The way to do this is to add a new attribute field, populate it with the characteristic you are trying to preserve, and then merge. In other words, you’re moving the distinguishing characteristic from the file name or file folder to the attribute table, so it is not lost, prior to merging.
For example, I have a list of basemap layers for the Oceanic countries of Australia, Paupa New Guinea, New Zealand and Samoa. I want to merge all of the like layers into one for the entire region, so I have an Oceanic road layer, an Oceanic river layer, etc. But before I do that, I want to add a country field and put the country name in there. This way if I need to select out just the Australian roads later, I can.
It’s pretty tedious to open each attribute table, add a field, and then run the field calculator to stick the country name in there. This task can be sped up considerably through the use of model builder. Model builder allows you to batch process ArcToolbox commands, and to chain commands so that the output of one tool into feeds into the input of the next tool. We will be able to add new “Country” fields to all our Australia layers at once, and immediately insert the value “Australia” into those fields, with a single click of mouse.
Let’s get started. Open up ArcCatalog and activate ArcToolbox. If you have never used model builder before, you will need to create a new toolbox to hold your model. Then, create a new model within that toolbox.
Now, drag the tools that you want to involve into the model screen. In this case, that’s “Add Field” and “Calculate Field.”
We’ll be starting with Add Field. It’s time to choose your input layer. By default, the tools dragged into model builder accept a single data layer as input, but you can change them to accept a list of layers (batch mode). To do this, right click on the Add Field tool to make the Input Table a variable. Setting something as a variable “exposes” it, which means it makes it editable independent of the rest of the settings that you will configure later. Model builder indicates a variable has been exposed by giving it its own symbol (an oval) in the model flowchart. The other settings remain hidden inside the tool symbol (rectangle).
Now, right click on the Input Table variable to bring up its Properties. Then, change it to a list of values.
The Input Table symbol will now change to a stack of ovals. If you double click on the stack, you will be able to cue up as many inputs as you like. The fastest way to generate a list of inputs is to drag them from windows explorer. Note: You can’t drag them from ArcCatalog because the input box maintains focus until you dismiss it.
You can navigate to the folder, and manually select all the .shp files. Or, even faster, you can do a file search for them. This gets any subfolders too! I’ve searched within my Australia folder.
If it works right, your stack of ovals will turn blue. Now it is time to set the rest of the Add Field settings. You can access them by double clicking on the Add Field rectangle. Here, you will see your list of settings. The Input Table column is filled in but the rest of the settings (field name, field type, etc) are grayed out, except the first row. Double click on that first row to edit the other settings. Here, input your field name (“Country”), field type (“TEXT”) and field length (“50″) just like you would when activating the tool from ArcToolbox. Note: If you leave the text field length blank, it defaults to 50.
Because you don’t have any of these parameters set as variables, the settings you enter here will propagate down through the entire list of inputs. When you finish, Add Field should be yellow and the output oval stack should be green. This indicates all of the settings are valid, and they are ready to run.
Next, configure the settings for the Calculate Field tool. You’ll want to set the input of this tool as the output of the Add Field tool. Just look at the name on the oval extending outwards from Add Field. (The name is generated from the first layer in your input list). In my case, the output of Add Field is called “admin_area (3)”. I’ve set that as the Input Table for Calculate Field. Then, use the dropdown list to select the Field Name you want to populate. Your new field, “Country”, will show up in the list of options because model builder runs the commands in order and knows you’re planning to add that field in the previous step. Finally, type the string you would like to use to populate the field in the Expression box, with quotes around it. In this case, “Australia.”
Now, you can run your entire model from the model menu, or one tool at a time by right clicking it. I like to do one tool at a time at first so I can be confident each step is working correctly. The model parameters will turn red while running, and will form drop shadows once they have finished.
Now, you can rerun the same model for the other 3 countries. All you need to change is the initial (blue) input list and the Expression string within Calculate Field. If you want to do even less babysitting, you can make the Expression a variable and do all four countries at once. And of course, if you want to add and calculate more fields, you’d just string them along down the chain.
Remember any ArcToolbox tool can be dragged onto Model Builder, so the potential uses are limited only by your imagination. For more information, see (both PFDs):
- Geoprocessing in ArcGIS (ESRI Tutorial)
- Model Builder: An Introduction (ESRI Conference Presentation)
Tags: attribute table, model builder