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FEMA Guides and Specs FIS -LIDAR/Scale/Contour question

FEMA Guides and Specs FIS -LIDAR/Scale/Contour question

FEMA Guides and Specs FIS -LIDAR/Scale/Contour question

(OP)
Hello all,

App J, section 4.1 of the FEMA Guides and specs requires the county to include the scale and contour interval of the source map data. However, the work I am looking at right now only states that 'the floodplain boundaries were delineated using a 1-meter cell size grid created from a county wide LIDAR".

So my question is when someone is talking about a 1-mete cell size grid, are they talking about the scale or contour interval or both? thanks!

RE: FEMA Guides and Specs FIS -LIDAR/Scale/Contour question

I believe it means that elevations were surveyed at 1 meter intervals in a grid pattern. That data can be used to generate contours at any contour interval you want, although the modeler probably used a DTM which was created from the LIDAR data and cut cross sections directly from that. Therefore, no contour map was used at all.

RE: FEMA Guides and Specs FIS -LIDAR/Scale/Contour question

Hopefully they were more specific on the source LIDAR data.  There could be multiple data sets in a given county.  If one does know the data origin, there will usually be accompanying metadata which should include the accuracy of the data.

RE: FEMA Guides and Specs FIS -LIDAR/Scale/Contour question

The grid is a derivative of the original data set, which is a LAS file (a whole bunch of points representing light returns). I'm not an expert, but basically, a grid with elevation cells of a given size (5 ft means it is a 5 ft by ft pixel with a particular elevation value, etc.) are capable of "holding" a contour of some interval. I don't know the math off-hand, but you could google the concept and see what you get.

In short, a X by X grid can be used to create contours at an interval of Y or greater.

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