BY G2 INTEGRATED SOLUTIONS
In the previous post we learned how to prepare pipeline polylines for load into a PODS ESRI Spatial or APDM geodatabase by adding ‘M’ values (measures / station values) to the vertices of the pipeline polylines. This is well and good, but before charging ahead, a primer on the pipeline centerline as modeled in the APDM and PODS ESRI Spatial is in order.
In both PODS ESRI Spatial and the APDM, PolylineM features representing the pipeline centerline are stored in a feature class called StationSeries. The centerline does only two things: it stores the spatial location of the pipeline, and it defines the linear measurement system. For this reason you won’t find any physical pipe information (e.g. pipe diameter) in the StationSeries centerline features. Think of the centerline as the spatial framework for all the features that comprise the pipeline.
In both the APDM and PODS ESRI Spatial, the pipeline centerline defines two systems of linear measurement (or linear referencing, to use the ESRI term). These are continuous measure andengineering stationing. Engineering stationing is what pipeliners actually use; continuous measure is a crutch for us GIS geeks. No doubt this seems redundant and unnecessarily complicated. But it turns out that limitations in ESRI’s linear referencing technology make it necessary. (More on this in a bit.) The two systems of linear referencing result in two distinct types of station series features:continuous, and engineering.
A continuous station series feature represents an unbroken segment of the centerline. Physically, a continuous station series corresponds to a continuous run of pipe, for instance, the pipe between two compressor or pumping stations. (And for those of you familiar with the PODS relational model, a continuous station series is a PODS route.) Continuous measure values on a continuous station series feature must be monotonically increasing (no gaps or overlaps, no duplicate values). A pipeline is composed of one or more continuous station series features. While a continuous station series feature can have no gaps internally, there can be gaps between continuous station series features.
A continuous station series is composed of one or more engineering station series. Spatially, a continuous station series feature must be completely covered by its corresponding engineering station series features, with no gaps. Furthermore, there can be no overlaps between separate engineering station series. In other words, the engineering station series that comprise a continuous station series must connect end-to-end.
Engineering station values must increase or decrease monotonically (relative to the parent continuous station series) within an engineering station series. However, there can be gaps or overlaps in engineering station values between adjacent engineering station series within a continuous station series. These gaps or overlaps in engineering stationing values are referred to by pipeliners as ‘station equations.’
The relationship between continuous measure and engineering stationing is extremely important. For a given continuous station series feature, the total continuous measure footage must be matched by the sum total footage of engineering stationing in the related engineering station series features.
Are you confused yet? A picture is worth a thousand words, so here goes:
The example above not only paints a pretty picture, it also illustrates why we need two systems of linear measurement. In a previous post I mentioned that ArcMap is smart enough to plot a feature on a pipeline even if I don’t know its X/Y, but as long as I know its measure value. Well, kind of…
When you specify the location of a linear feature on a pipeline in ArcMap using measure values, you supply the begin and end measure and the ID (singular) of the polyline on which it occurs. This means you can’t post a linear feature that starts on one polyline and ends on another. This is true even if the polylines in question are connected end-to-end, like our engineering station series.
Suppose I have a gas HCA segment that starts on engineering series 1000 at station 300 and ends on engineering series 2000 at station 500. Oops. Sorry, Charlie. In out-of-the-box ArcMap, no can do!
Hence, continuous measure and continuous station series. If I use continuous measure values to post my gas HCA feature, no problem. My gas HCA segment begins at measure 300 and ends at measure 450 on continuous station series A. ArcMap can post this because the measure range is on one polyline feature.
In our hypothetical raw pipelines layer we didn’t have stationing to begin with, so no worries about gaps or overlaps in engineering stationing. In our happy little world, continuous measure = engineering stationing. YAY! Nonetheless, to remain model compliant, we must load both a continuous and an engineering station series for each of our pipelines. In effect, we load the same polyline into the StationSeries feature class twice. And that’s just what we’ll do, in the next post!