Merging Contours

I created 2' contours for all of Garrett County, MD and have them all in tiles provided to us with the LiDAR data. When I generated the contours from the 1m DEM, I was unable to do it county-wide so I broke it down into 3 regions (top right, top left, and bottom). I also have 17 of what I'm calling "groups" of tiles (screenshot) that fit into one or the other region (except for group 11 ... read on to the 3rd paragraph). I had some overlap in all the regions to be sure that I had good data covering those boundaries. This actually causes some problems when I go to merge contours back together.

Justification for the merge involves the need to create 10' contours for the entire County (and 20', 50', 100' etc.). Since these areas overlapped, I wanted to merge lines from each of the 3 regions. I discovered that doing a dissolve works well for the boundaries that match at an edge (after doing a merge) but if the contours themselves overlap, the dissolve keeps both lines. I tried setting the tolerance to about 2' in the Environment settings because I read that that works but either 2' was not high enough (I was hesitant to go any further) or it just doesn't work.

I had a particular problem in group 11 because the top right region didn't completely fill all the tiles in that group and neither did the bottom region. So I had to erase from both the top and bottom and fill the top right region in with the individual tiles and then it all worked fine. Next step was to merge the 3 regions together and then dissolve. It worked great because at that point, the contours only matched at an edge and the dissolve didn't have any problem. Incidentally unchecking multipart and checking split lines was key here.

The steps (in summary) are clip, erase (if contours overlap), merge, and dissolve. the dissolve reconnects the lines that should be connected and using split lines and NOT having any multipart features prevents connecting any non-coincident lines.

group 11 tiles

contour_tiles.JPG35.25 KB