Keep Your Curbed Islands Afloat

Grading in curbed islands can be a time consuming task and they often need to be changed based on the overall grading changes. Whether it’s a curbed island in a parking lot or a curbed median on a boulevard, secondary corridors give you another option to keep your islands dynamic.

The first thing to understand is that you shouldn’t get hung up on only having (1) proposed surface or what I like to call PG for proposed grading. You want to have your final PG as a combination of sub-PG surfaces, or WORK-PGs. The additional sub-PG surfaces allow you to grade to intermediate proposed grades on your site or to obtain elevations from those sub-PG surfaces. The Corridor surface would be considered one of those sub-PG surfaces. I will usually have (1) main Corridor surface that includes all my Streets, and Alleys. Then I’ll have multiple Corridor surfaces for things like swales and in this case, curbed islands. All these sub-PG surfaces are then pasted into the overall PG. Now in this example, I have a Work-PG that has my main Corridor, feature lines, and some manual contours. Then my overall PG will have my Work-PG surface, and the island Corridor surfaces pasted. You may be asking, “why not have everything pasted into one PG?” Well, that’s the trick. You need your island Corridors to be dynamic and change as changes occur with either the street Corridor or parking lot grades. In other words, if you have only one PG surface and you decide to get your elevations or “surface profile” from the PG, then you go and paste an island corridor surface into that PG, those elevations become static at that location.

Confused? Think of your curbed island as a boat out at sea. Your WORK-PG is the “sea”. As the waves, or “grades”, change so does your island. Make sense?

First, you need to have some curb assemblies. I have one RIGHT and one LEFT. The direction of your alignment will dictate which one to use. Note: You can add additional slope links if you would like. I prefer them simple to allow for adding addition freehand berming or other grades after they’re pasted into the overall PG.


Next, create an alignment along the edge of the island. I’ll usually use an alignment style that is set to NO-PLOT or NO-SHOW.


You want to make sure you have your WORK-PG up to date to this point. Then, you need to create a Surface Profile from the WORK-PG surface. Note: There is no need to create a profile view unless for your own reassurance.


Now we can create a Corridor for the island.


You can see here that the PARKING-ISLAND-1 Corridor is now draped over the PG-WORK surface.


The next step is to create a PARKING-ISLAND-1 corridor surface.


This image shows the PARKING-ISLAND-1 surface and the WORK-PG surface together but separate. Any changes made to the WORK-PG will dynamically update the PARKING-ISLAND-1 surface.


You can now Paste the PARKING-ISLAND-1 surface into the PG surface. NOT the WORK-PG. Remember…, the island needs to stay “afloat”. You also need to realize that the order of Pasting surfaces does matter. So in this case I have the WORK-PG pasted first, then the PARKING-ISLAND-1 surface.


This image shows the resulting PG.


The second example will use the same process for a median in a boulevard. However, the difference is that the main Corridor surface can be used for the surface profile. I know some are thinking “Why not just use an assembly in the corridor itself to create the median?” Well, you can. However, the problem comes when you have “rounded” ends at the medians. If you’ve tried it, you’ll know exactly what I’m talking about. Since the assemblies are applied like sections that run perpendicular to the centerline, the “rounded” ends won’t be modeled correctly. So the answer is to have the main Corridor designed as though there is no median. Then, add the median as another Corridor that stays dynamic with the dynamic changes of the main Corridor.

First, need the alignment defined.


Then, we’ll create a Surface Profile.


Next, is the Corridor.


This image shows the Main Corridor and also the Median Corridor draped on top.


Create a MEDIAN-1 surface from the MEDIAN-1 Corridor and then paste that surface into your PG.


This image shows the resulting PG with both the main CORRIDOR surface and the MEDIAN-1 surface pasted in. Again, make sure the main Corridor surface is pasted first.


Keep in mind, there are many other things you can do with these islands and still keep them dynamic. One might be to have a slope link on your assembly that grades up to a specific height relative to the top of curb or have your typical offset at 2%, or what ever it might be. Or just keep it simple like these examples and that will allow you to have flexibility for “manual” additions inside the islands or medians.

Just keep an open mind, and make things fun while making things more efficient.

Hopefully this is a “little” more insightful than my last post about the mouse battery.



  1. Mark Spatz says:

    Good post! This needs to become apart of our standard working process. I just started working with one of our offices that is pro-Microstation and this is how they do medians in that program. Except in Civil 3D everything is dynamic whereas they have to re-sample anytime anything changes. That caught there eye…

    They also are liking Annotative text. I guess it is the little things…

  2. Tony says:

    Great post. One thing I like to do when I have limited time in grading is just create a feature line around the curb island. Then use the get elevation from surface feature for my feature line elevations. Then use the offset feature to create my TC. I sometimes have to fix a few tin lines but it works. Of course I only do this when I have just about finalized my PG surface. I will definitely have to try this out since I am working on a street project right now. I have done something somewhat similar to this. Anyways great post.