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

[biker317]

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!

 

[cvg]

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.

 

[TerryScan]

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.

 

[trevortim]

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.