[GIS] Drawing irregularly shaped boundary polygon around set of LAS points using ArcMap

arcgis-10.1arcgis-desktopdemlaslidar

I am working with a set of LAS data which is irregularly shaped, picture a starfish as the point coverage, surrounding the starfish may be water on one side, may be more land elsewhere but no points were collected for it. In fact most of the areas around this starfish of points is land without points collected, and inside the starfish shape is a little lake.

I want to make a DEM of just the area where points were collected, and there are many ways of doing this, but most will interpolate all the no-point areas inbetween "arms" of the starfish data point pattern. Making the DEM by using a Terrain Dataset allows for the input of a bounding shape, to theoretically omit no-point areas from being interpolated and only geoprocessing inside of the boundary shape.

However, I have difficulty coming up with a precise bounding polygon of the outside edge of the starfish to use as my geoprocessing limiting extent.

I found a process online in a blog by Clayton Crawford which was picked up by Esri and used as their help file for the process of turning LAS to DEM's, but it involves downloading a custom VBA script and several steps which went a bit over my head.

Does anyone have a suggestion on how to get that starfish pattern of LAS datapoints inside a bounding starfish-shaped polygon, without the use of VBA, or alternatively if there's a way to go right from LAS to DEM raster with no-point areas end up as NoData?

I'm using ArcGIS 10.1 for Desktop.

Best Answer

How to create the Concave Hull around an irregular point cloud in Arc10:

You have an input point cloud (for better performance > MULTIPOINT).
Create a TIN: 3D Analyst > Data Management > TIN > Create TIN.
Delineate TIN data area with a max_edge_length as parameter (this depends on the distance between your input points, e.g. I'm using 10m) and parameter "PERIMETER_ONLY": 3D Analyst > Data Management > TIN > Delineate TIN Data Area.
Use the tool 3D Analyst > Conversion > From TIN > TIN Domain to save a polyline or polygon item outlining your TIN.

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ArcGIS Polygon Sizes – Dividing Polygon into Specific Sizes Using ArcGIS Desktop

This problem has many valid solutions. One of them works a little like your description, but instead of slicing the polygons at "random" locations you can do it purposefully in a way designed to minimize the amount of computation.

Here is the basic algorithm. Its input consists of any plane sweep direction, a polygon P of nonzero area, a target area a between zero and the polygon's area, and a nonnegative threshold t (in units of area). Its purpose is to split P with a line perpendicular to the sweep direction into two parts, one to the right of the line and the other to the left of the line, such that the difference between the righthand area and the target area a is no greater than t.

Let L be any oriented line perpendicular to the sweep direction. Define f(L) to be the area of P found at the right of L, minus a. In these terms the task is to find a zero of f. Because f is unlikely to be differentiable, but is continuous, use either a bisection method, the secant method, or--my favorite--Brent's method. All are simple and guaranteed to converge. Use t for the convergence tolerance for the argument.

That's it. Let's consider what goes into coding this. The root finding is routine--you can use a generic chunk of code for it--so the GIS work comes down to coding f. Doing so requires

1.  Splitting the polygon by a line.
2.  Computing the area of the piece(s) to the right of the line.

Both operations are implemented in almost any vector-based GIS. If not, you can replace the line by a very large rectangle representing the half-plane to the right of the line. Step 1 becomes

1'. Clip the polygon to the rectangle.

That is a really basic operation.

To get started with root finding, you need to find an interval in which the zero of f is guaranteed to lie. This is easy: project the polygon's envelope ("bounding box") into the direction of the line sweep. The projection is the interval you want.

This question has a long history. I implemented this algorithm for ArcView 3.x long ago and described it many times in the old ESRI user forums. Google

huber split polygon site:forums.esri.com

for discussions, links to code, enhancements and variations (such as splitting polygons into parts of desired sizes which are as compact as possible) and algorithms for raster data.

Here is what the continental US states look like (in an equal area projection) with the bottom third of each state shaded. Evidently the sweep direction was vertical.

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[GIS] Creating boundary polygon shapefile from set of LAS files using ArcGIS Desktop

After looking into both solutions proposed previously, I stumbled upon a dead simple way to do this.

Use 3D Analyst Tools > Conversion > From File > Point File Information.
In the dialog box, select Folders in the "Browse For:" drop-down menu. Click on the Folder icon and select the directory where your LAS files reside.
Enter the name and desired location for the Output Feature Class.
Specify the Input File Format (in this case, LAS).
Enter file suffix of the files you want to include in the process (in this case, las).
Enter the Input Coordinate System. It says that this is optional, but it is only optional if the coordinate system is specified correctly in the LAS files. Personally, I would look up the correct information from the metadata and enter it here.
Click Ok and wait for the process to finish.

Now you have a polygon shapefile consisting of the footprints of each and every LAS file in the directory.

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ArcGIS Polygon Sizes – Dividing Polygon into Specific Sizes Using ArcGIS Desktop

[GIS] Creating boundary polygon shapefile from set of LAS files using ArcGIS Desktop

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