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Offset valves

October 22 2008 | 0 comments
Categories: ArcGIS Methods, Cartographic Design, Data Modeling, Map Data, Symbology

We have large gear-operated valves in our water system (16" and up) that are "laid over" underground (parallel to the surface) because they would break grade if installed standing upright. When gathering GPS point data, it quickly became clear that, unlike all of our smaller valves - where the water main is directly under the surface valvebox lid - the at-grade valvebox lids of these "laid-over" valves are offset from the water main beneath by as much as 4 to 5 feet.

I have experimented with a "barbell" shaped icon to represent this situation in a manner that would allow me to properly represent the sub-surface location of the main and have found that I must manually rotate these symbols to their correct location...it's a laborious task.

Any thoughts about how to address this situation cartographically?

Mapping Center Answer:

This was a long time in coming, as we had actually done a good bit of work and consultation with a group of water utility experts. 

First, this was a really interesting problem to consider.  We looked at how such conditions might traditionally have been mapped (ink & paper methods) at engineering and some smaller GIS scales.  I found that the depiction of water service meters is potentially similar in terms of the sort of cartographic abstraction that you might employ.  In many cases the water meters are automatically placed with in the parcel that is the basis for service.  That works well in the single family house neighborhoods.  It's a bit harder for multi-unit parcels--the solution there was to make a T and then a spur off the top of the T for each meter.  Sometimes you had to zoom in as far as 1:200 or even 1:100 to see each meter clearly.

For your valves, being on 16"+ mains, it's a good bit more important to see status than for a mere meter.  Thus, it's very important to show the kind of valve, and very likely at smaller scales than 1:200.  You can do this with just a symbol, or with a symbol and geometry.  Either way, I would suggest figuring out the smallest map scale you want to clearly see the state of these valves.  Based on that, here is a bit more on each method:

  • Just a Symbol:  In this case you you've got to show the type of valve, the state (open / closed), that you have a special kind of valve box (under surface), and that it is offset a specific distance and direction.  The latter two can be handled in the label. The valvebox maybe just a solid (versus hollow box) and the valve type symbol can be fairly standard (no need to rotate), but colored based on open or closed.  The whole thing needs to be large enough to be easily seen, so 24-32 pts would be a good start.  You likely don't have a great many of these valves, so the additional labeling shouldn't wreak havoc accross any map sheet or screen extent.
  • Custom Geometry:  Here, the geometry, regardless of real-world distances, must be large enough to be easily seen on your maps at the smallest working scale.  That means you will likely have to significantly exaggerate the geometry (no different, conceptually, than the way I described typifying how meters got drawn above).  You would no longer need to label the direction of the offset, but the distance would be an issue.  You would also still need a symbol that indicated the special box and state of the valve.  However, this symbol could be smaller (the offset would make up for the map recognition task that the larger symbol compensates for above). 

After working with these maps for a couple of weeks, I am totally unconvinced of the need to rotate valve symbols.  It's very subtle, requiring a map reader to be focused as they interpret the map, it's a lot of work, as you point out, and there is no operational value that a field crew won't immediately percieve once on site.  That said, I'm not recommending that the angle of orientation should not be tracked as an attribute.  I think as more maps in water utilities become digital it will be easy to pop-up the relevant attributes of any valve you click on--accomplishing the mission in a more meaningful way.

So, in the end, it's about what you want your maps to look like and how you want to train your field staff to read the maps.  I think the custom geometry solution is more work, but your maps may look more like they do now, while just using a symbol and additional labeling information saves work, but may create a different look to the map. 

I also think the map scales that you would typically use this information at are an interesting consideration.  These are not typical valves in that they apparently can have a broad effect on water delivery and so there may be a case for just using a symbol on a smaller scale map (versus the field maps) to show the impact of these valves on your water distribution network features.

Last, you didn't say you had to make your map work for black & white printing, so I chose to ignore that possibility.  If that is the case, I would suggest an even larger symbol (36-48 points) or using a larger map scale to allow you to create a visually distinctive symbol where state can be denoted by something like a hollow vs. filled barbell symbol.

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