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Symbology when features are part of the points of interest layer and areas of interest layer

January 27 2011 | 0 comments
Categories: Symbology

I am creating several maps that contain both areas of interest, and points of interest. Is there any standard for mapping features that are considered both an area of interest, and a point of interest?
Here are a couple examples:
I have the airport areas symbolized by color, and I also have an “airplane” icon where the terminal is located. Should I show both the area and the point on the map? If I show both features, should I try to table the point on the map, or should I try to label the area?
Second, I need to deal with the green space around school buildings. The original area of interest file that I had has a mixture of the green space around schools either being nonexistent, displaying the green space around the school (where the fields are), and containing the entire school property. Is there any standard on school area for an area of interest layer? Next, like the airport layers, I will need to know if I should display both school points and/or areas, and I will need to know If I should label off the points or areas.

On the attached map, you will notice:

The Airports are currently symbolized with the area of interest and the points of interest. Labels are based off the area.

The area for the Marshall High School [located on the right side of larger map] includes only the green area of the school property. The Tracy High School [located in the lower left of the smaller map] includes the green space, and the building area. Some of the smaller elementary schools are not even part of the area of interest. Which way is the standard for displaying school areas on a city map?


Mapping Center Answer:

To help, you can think of this as an issue of redundancy.  Showing both an area as a filled polygon (like an airport area) and a mimetic point symbol (like an airplane icon) is redundant because both indicate that this is where there is an airport.  Labeling both is also redundant.  However, showing the area and showing a real feature, such as the school building, is not because the area indicates the extent of the airport and the building indicates a feature within that extent.  So you can eliminate the point symbol in areas that are shown and labeled as the school.

Then the question becomes, as you suggest, what do you use as the extent of the area.  You used an example of green spaces around schools versus school properties.  To help with this issue, you can think of those two things as entirely different kinds of features.  One is a land cover type -- green grass or trees, and the other is a land use type -- the area used for a school.  It is entirely possble that you would have green land cover in areas that are not schools (parks and other open spaces) so to be consistent across the map, you would simply show all green land cover areas.  However, if instead you want to show all the ares that are used for schools, then you would focus on finding the extent of the school area.

Land cover is easier to map than land use, because it is something that is visible (green trees or grass).  But sometimes land use can be hard to map if there are no distinct visible boundaries or other clues as to the extent of the feature (e.g., parcel boundaries or zones on a zoning map).  So you have to do your best - e.g., use rule-based image interpretation, or use ancillary data (like a parcel or zoning map).

Two more things to think about -- the size of the feature relative to the map scale, and attempting to show both land cover and land use.  And finally, I will answer your question about labeling.

In terms of the size of the feature relative to the map scale, you may want to show area fills for the larger features (e.g., larger schools), and then show point features for the smaller schools.  If the polygons for the smaller schools are too small for the reader to see the fill color, then that is a good indication that a point feature would work better for those smaller features.  It is OK to combine area and point symbol representations for the same feature on the same map -- just be sure to include both in your legend.

A simple fix for your map would be to use a different color for the school areas (a good choice would be pink because the university is also that color and that is also an educational features) and use green for the park areas.  I suggest thsi because it appears that you are only mapping land use nd not land cover. Whatever color you use for the fill of the school areas, you could also use for the point symbols for the smaller schools.  Again this promotes visual association between ALL the schools.

In terms of attempting to show both land cover and land use, you will have a difficult or impossible time showing both using colored area fill symbols.  You COULD use a color fill for one and a pattern fill for the other, or you could use a color fill for one and a different outline for the other.  Either of these approaches are best reserved for a case in which there are only a few categories for the pattern fill or outlined features.  For example, if all you have are schools and airports as land use categories, then you could use special outlines for those two types of features, and you could still show all green spaces.

Which brings me to the final point about labeling.  We have found that we have better label placement control when we create centroids from the polygons and use those to label the areas.  You can also shift the location of the point feature to assure better label placement.  What this means is that for the polygons you want to label, you can create point features and label those.  If you use the outline approach described above, then the label can be the same hue as the outline thus assuring a direct visual association between the polygon that is outlined and its label.

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