I once had a job interview where my future boss asked me the title of this post, “Why are Manholes Round“. I think the question was designed to see if I could “Think on my feet” or solve the problem under pressure. Fortunately for me, I had previously worked closely with civil engineers, and knew the answer.
In mapping, the user can shape manholes or color them any way desired. That being said, appropriate map symbology is important for quick feature identification. Esri’s ArcMap has numerous OOTB symbol sets that may be quickly chosen and leveraged. There are libraries that may be added to those readily available by default. To add/research these libraries, follow these simple instructions:
- Within ArcMap, right-click on the layer (within Table of Contents) and choose properties. The Layer Properties dialog appears (Figure 1).
- Click on the symbol (currently a small brown circle) button. The Symbol Selector dialog appears (Figure 2).
- Although some of the shown default Esri symbology would probably be sufficient, click on the Style References button (in the lower right hand corner). The Style References dialog appears (Figure 3).
- There are a plethora of symbols that we can load as default. In this example, we are loading the Utilities symbol set. After clicking the OK button, we see the extra symbols added to the bottom of the list of list. We can click on the desired Manhole symbol, and click OK (Figure 4).
- There is another way we could have quickly searched for manhole symbols within all the OOTB symbols within all the Style References. The user can input in this case “manhole”, and all the symbol options appear (Figure 5).
The user has the ability to leverage existing fields/attributes to symbolize the layer differently based upon their populated value. The procedure is simple:
- Go to Layer Properties and the Symbology tab (Figure 6).
- Choose Categories (Unique Values highlights automatically).
- Specify the field to symbolize off under Value Field. In this case, I have chosen Subtype. A list of unique values appears.
- Click the symbol adjacent to each value to change to an appropriate symbol as shown in the previous steps.
- If desired, click the up/down arrows (at right) to adjust the order of appearance in the Table of Contents and Legend.
Use of existing symbols is encouraged for faster data rendering. There are times when the provided system symbols are not sufficient. In a previous post, I discussed the necessary act of motivating the often unmotivated seasoned veteran.
One of the ways to do this is to replicate previous symbology from legacy system maps (i.e., CAD). In addition to what I have outlined above, one can also combine symbols in some fashion to produce the desired results. This is conducted as follows:
- From the Symbol Selector window (Figure 4), click on the Edit Symbol button. The Symbol Property Editor window appears (Figure 7).
- In the Layers window, the user sees the original symbol, and can change it to the desired shape. I am changing it to a pentagon by first changing the dropdown from Simple Marker Symbol to Character Marker Symbol and selecting the symbol in the available chooses (Figure 8). Note that I have also increased the default size and increased the preview zoom.
- We now want to add the text “MH” in the center of the pentagon. We click the Add Layer button in the lower left, switch the font to Arial Black, choose the M letter, and reduce the size as necessary (Figure 9).
- It is now necessary to move the “M” to the left to make room for the “H” (Figure 10). With the “M” layer selected, decrease the X-Offset (in this case it is -2.5).
- Repeat the same steps for the “H”, but this time add a positive X-Offset to move it to the right (Figure 11).
- Lastly, it is often necessary to put a background on the symbol, so the symbol shows up on top of orthophotography. We can add a new layer (solid pentagon) just like in the previous cases and move it to the bottom of the stack (Figure 12).
It is my opinion that symbols should be as small as possible and simple. Small symbols aid in digitizing and minimize obscuring layers/information. A word of caution though after working with clients for several years. I have seen more disagreement over symbols and color than just about any other subject. Now go have fun, and be flexible!
So why are manholes round? The lids are heavy (over 100 lbs). Manholes can be quite deep as well. Manhole lids are round, so they can never fall into the hole in any position, which cannot be said for squares or rectangles.