# comparing two TINs

i have the following two files:

1) LiDAR las file1

2) Let's give it a name LiDAR las file2, contains the same file1 data, where number of points randomly removed.

So if file1 contains X number of points, file2 would contain Y number of points, where Y is less than X.

Now, i am using ArcGIS "LAS to Multipoint" to create from both files above their shape counterparts. Next i am creating two empty TIN(triangulated irregular network) files using ArcGIS. Then i edit both TINs, let's say tin1 and tin2, with the appropriate input feature clasess, i.e. file1 to tin1 and file2 to tin2.

Is there any tool/package/library or any software that would let me calculate the RMSE(root mean square error) or any difference information that would give me information on the accuracy of the second file, i.e. file2, with respect to file1? I tried to use the TIN difference but it gives system error and cannot obtain the result. I would really appreciate if anyone could show how to obtain accuracy information and interpretation of the results obtained...

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Please provide more details on what you are trying to model. I think tool you choose should be based on what are you trying to model: bare ground, tree canopy, power line catenary, etc. –  Kirk Kuykendall Feb 2 '11 at 17:19
Also, maybe this should be more generally stated as "How do I compare two Point Clouds". en.wikipedia.org/wiki/Point_cloud –  Kirk Kuykendall Feb 2 '11 at 17:25

### Approach

You either need to solve the system problem or perform a substitute or work-around comparison. Because you don't provide details about the error, I conclude you're looking for a work-around.

### Solution

Consider converting both TINs to raster format, where a little map algebra does the rest. For example, the RMS measure of difference between the TINs is obtained by subtracting one grid from the other, squaring the result, and taking the square root of its average value. The answer depends on the cellsize (and grid origin) you use, but as the cellsize grows small, the answer will converge to the one you would obtain using the TINs directly.

### Tools

There are many tools and libraries to perform such comparisons. They are the bread-and-butter operations of raster GIS packages like Spatial Analyst, IDRISI, and GRASS.

### Accuracy

Note that no such procedure, whether applied to the TINs or some other representation, can estimate accuracy (which concerns agreement between the TIN and reality): it can only compare one data set to the other. Both could be hugely inaccurate, yet in close agreement.

### Interpretation

There is nothing unusual about interpreting these results: one inspects both maps and the difference map, looks for outlying values in their differences, characterizes the statistical distribution of the differences, assesses the spatial correlation of the differences, identifies locations where clusters of high or low differences occur, etc., just as always.

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Hi whuber. I hope you don't mind that I have added this good answer as complement to one of my answers in a related thread. Thanks. –  Andre Silva Aug 27 '13 at 17:32
@Andre Thank you for directing my attention to your extensive and interesting answer there (+1 to it). –  whuber Aug 27 '13 at 18:17

You can do this with lasheight (README) or lascontrol (README) of LAStools. With lasheight you would need to create a TIN from the points of one file, and then compute the relative heights (option '-replace_z') of the points from the other file against it (and vice-versa). With lascontrol you would use the LAS points of one file to compute the TIN and turn the LAS points from the other file into CSV control points. Contact me (email is in the README) if you need details on how to do this.

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We appreciate replies from vendors and other commercially interested parties, Martin, but in such cases to retain credibility it's crucial that you identify your connection with the solution you are recommending. –  whuber Feb 13 '12 at 15:46