Part 1 - Whats Is A Geoprocessing Task And Why Should I Care?

So, lets begin this discussion of Geoprocessing with ArcGIS Server that I promised. I guess the first question is what is geoprocessing and why should I even care?

The big promise of ArcGIS Server has always been this: "Whatever you can do in ArcMap and ArcCatalog, you can now do with ArcGIS Server". I have often heard ESRI reps say 'You can even rebuild ArcMap if you want to'. Of course, no one would ever want to do such a thing, but wouldn't it be nice to have all that functionality available to you in a web based application?

In the old days of ArcIMS and MapObjects IMS we were pretty limited in what types of geoprocessing we could do on the web. I can recall all sorts of crazy tricks with buffers that we used to write to really push ArcIMS to its limits. But, at the end of the day, it was still just a buffer, and there is only so far it will take you. How about a select by location, or a union, or a clip? Sorry folks, you were out of luck if you needed to do that on the web. But then, ArcGIS Server came around, and suddenly doing real geoprocessing became a reality.

So, what is a geoprocessing task? In short, it is a set of server side tools called from a web based client. Users create a model using Model builder, and then publish that model to the web as a geoprocessing task. The output parameters of the model are then streamlined to your web application.

Lets take for example a recent Flex application that I wrote (Check it out here). The purpose of this application is to help citizens locate their property and then return information about the zoning on that property. It sounds easy enough, right? I'll bet you are thinking, why not just do an Identify, and return results from the zoning layer? Ah, if it were only that easy.

There can be many, multiple types of zoning found on a single parcel, so a simple identify by point will not work. We need to do a spatial analysis to get the property boundary that the map click point or address point resides in. Once we get the property boundary, we need to do a second spatial analysis to determine which zoning layers intersect that property boundary.

So, how can we use a geoprocessing task to help solve this problem? I started by simply going into ArcMap and writing down the steps that it takes to get the results I need. Then, I used model builder to string together these tasks into a single workflow. Finally, I published this model to ArcGIS Server so I could access it from the web.

The, the final product takes in a point, selects the containing parcel for the point, and returns the zoning types that are present in the parcel.

Now, this is a pretty straightforward example. We could take this example much further. We could actually clip that zoning layer and determine precise percentages of each zoning type on that parcel. For parcels with multiple structures, we could determine the zoning district for each structure. List goes on.

This scenario, while certainly not impossible to do by conventional means (thinking FIND, QUERY, and IDENTIFY tasks), becomes much, much easier to solve by using a geoprocessing task. It also combines all the steps into one task. Instead of writing a complex combination of code to perform these steps in javascript or actionscript (flex) on the client, I have created a new, reusable tool that does all of these steps for me, serverside.

I hope this clears up what a geoprocessing task actually is and why you should care. Next up in the series, we'll tackle the technical aspects of actually creating and executing one from your web application.