Where'd My Property Go: Finding data during Splits and Merges

One of the challenges when working with geospatial systems is managing the data attributes when the object is split or combined with another. For example, if I have two parcels that are joining, and they have different Assessor Parcel Numbers (APNs), how do I get just one new APN from the 2 previous APNs. And what happens to the area field? Or when I split a parcel, how does the system name the 2 new parcels? More importantly, that about the ID field that serves as a key to link to other databases. The answer is that you can pretty much set it to do what you want. AutoCAD Map incorporated split and merge tools so that you can manage them. The following example should give you an idea of how to get started. The important thing is that like most things GIS, putting some thought into it prior to trying to do the work will give you the best results. In other words, design is very important.

My example will use a proposed forests data set. I have attributes for the proposed name, the area, perimeter, and the area in square kilometers and hectares. During the join, I want to rename to proposed forest, and update the area and perimeter fields.

NOTE: As I go through this, keep in mind the terminology gets wonky. The same words can be used to describe multiple elements of these objects. For example, the attributes of a feature can be called attributes, properties, fields and columns (to us database geeks), and the properties describing the said attributes, such as field size and type, can all use the same names. So, try not to read too much in the wording and I’ll try to match AutoCAD Map’s terminology.

To set the Split and Merge Rules, I will highlight the target data set in the Display Manager and open the Data Table.



Once I get the Data Table open, I’ll select the Options, and select Set Split and Merge Rules.




At that point, I’ll get the Split and Merge Rules dialog box.

At the left of the box, I get a list of all of the feature properties of the selected data set (attributes or database columns). As I select each of these properties, I get the various attributes of that property. It identifies whether the property is an Identifier, the data type of the property, whether it is autogenerated, read-only or nullable. In addition, I can set Split and Merge Rules for each property attribute. Keep in mind, the available Split and Merge Rules are context sensitive based on the data type (it’s a little tough to sum text fields).

The data set I’m using is an ESRI shape file, so there are certain feature properties that are inherent because of the type of data set. The FeatID is autogenerated and read only, so I won’t be able to set any rules for this one.

My ID field is an identifier for the individual forest polygons. I’ll set my Merge Rule to Empty. When merging, I’m going to create a new and distinct record from the previous record. This is a business rule I’ve decided upon so I can keep a history of forest proposals, even if they are not actually implemented.

My area field shows the area of the polygon. I could add the polygons, but for better results I can use an expression to calculate the actual area of the result. To do this, I set my Split rule to Calculation, and select the Expression Builder button (next to the Exporession box). I’ll select the Area2D from the Geometric pull down,


And then Geometry from the Property pulldown.


The resulting expression, Area2D(Geometry) , will calculate the area of the new polygon (if you know the expression, you could just type it in the box rather than going through the expression builder – but if I had done that, you wouldn’t have seen it, right?). That expression will go into both the split and merge rules.
My next feature property is the Perimeter, and guess what? There’s a calculation for that as well. Select Length2D in the Geometric pull down to get this expression: Length2D(Geometry)
On the Name feature property, I will generally not use the existing names – again a business decision. There are cases where I would want to keep one of the names (Using the FirstSelected or LastSelected rule) or concatenate the two names, just not for this case. In my example, I would need to add the name manually after doing the merge (or split).
My next feature property is AREASQKM, or the area in square kilomters. I can use the same expression as before, but include the conversion to square kilometers, giving me this expression: Area2D(Geometry)*0.00000009290304. Again this will apply to both split and merge.
My last standard property is HECTARES, which is the area in hectares. This will match the previous bit with the appropriate factor: Area2D(Geometry)*0.000009290304

Of course the last entry is Geometry, and you can’t use rules on geometry,