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I have a tendency to map in great detail…even when it is unwarranted or relates to a largely inconsequential stratigraphic situation. This problem is proportional to the quality of the base imagery that I have or the intrigue-level of the units in the field. However, in my quest to lead the effort to develop a surficial geologic map of 10,000 sq. km. of dirt in Clark County in a compressed time frame (see: http://nd2mp.blogspot.com/), I am learning that it is ok not to sweat the details, as long as you explain what comprises the mapped units. One thing that we have learned is that it is essential to develop an agreed-upon minimum map unit area (mmu). That is, the smallest polygon that is mappable at the chosen scale.

As far as I can tell, geologists are not very keen on the mmu, whereas the dirt mappers in the NRCS have codified the concept in their considerably more standardized procedure.
We have adopted a visual approach that is based on the basic legibility of a polygon at a specific scale, and have concluded that 10 hectares is a good minimum value to start with. 10 ha is 100,000 square meters…or a square that is approximately 316 meters on a side. Sounds kinda big, looks quite big in the field, and certainly looks mappable at 1:24,000. However, if you zoom out to 1:100,000, the story changes.
The map below shows a random area in the county at 1:24,000. A selection of polygons is labeled with respect to size in hectares.
mmu24kwAnno.jpg

Sure, those all look totally mappable, right? Well, not so much. Check out the area when outlined in a 100k map:
mmu100k.jpg
Now the story is different. Not only are the small polygons bordering on illegible, the scale of the task of mapping such small polys consistently in a reasonable amount of time is impractical without a huge expenditure of time.
There are some side effects of eliminating units below a certain size threshold. One is data loss. That will be handled by preserving the small polys as points. Thus their locations will be stored as will their attributes. This solution can also facilitate the mapping process by flagging those polys that need to be absorbed into larger, surrounding or adjacent ones. One other problem is the case of high-standing inselbergs. Some of these are very tiny, but protrude several meters above the surrounding surficial deposits. Thus, their omission is particularly notable when in the field. For example, in the photo below, the fairly conspicuous cluster of red sandstone inselbergs has an areal extent of approximately 10 ha.
IMGP2448edt.jpg
Not mapping such a feature may seem like a total affront to the sensibility of a geologist, but there will be a point there in the dataset that indicates an awareness of the feature’s existence.
What do you think? Do you know of a standard mmu? Does 10 ha look to big?

Posted via email from Fresh Geologic Froth

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2 Comments

  1. I suppose this is one of the great benefits of going to electronic data sets. Such tiny bits are inconsequential and basically invisible at the small scale, but zoom to a larger scale, and they become more important and more defined. On a small scale dead tree map, probably they’re best just dropped.

    And with respect to your closing question, I don’t think it can be answered without the context of map scale and total areal coverage. On the old 1:250,000 quads, 10 ha would be invisible; the same area would be quite visible on the 1:62,500 or 1:24,000 quads. If the units were important and limited in number, I’d probably include them. If they were only a slight variation on a dominant theme, I might ignore them. If there were many small outcrops within a larger area dominated by something else, I might map the larger area as “X with scattered occurrences of Y.” I don’t think you could reasonably determine what the MMU might be until you’ve decided what your primary format/scale will be.

    And of course there’s always the notes in the key, if you decide to avoid clutter in the graphic presentation.

    • All good points. The scale of the map is to be between 100 and 150 k. Ironically, this is driven in part by plotter dimensions, but even more so by the fact that the map is to be compiled within the next few months. Ideally, the digital data set could dynamic such that it could fluidly generalize as you zoom in or out. This can be approximately achieved with tiling multiple data sets compiled at different scales…however, these data sets don’t exist.

      As for ‘special cases’ of important outcrops or key units that is where the point data comes in…so the data are not lost. Rest assured, however, that I will be struggling not to map what I consider key units.

      After I posted the map, I immediately began to reconsider the 10 ha threshold. I will follow up with more about the scale of the effort and the scale of the map.

      Thanks for commenting!


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