Introduction
The purpose of our second exercise was to take the data that we gathered in the first exercise with the survey and create a terrain model in ArcGIS. In this exercise we reformatted our data so we could put it into ArcGIS and then looked for any areas in our survey that could be improved upon. We would then resurvey the areas needed and create a finalized terrain model.
We had to overcome some different challenges in being able to create a working terrain model from the data we collected. As well as update and format our data we needed to make sure that our model was close enough to the real thing. We needed to make sure that each feature was properly represented and the resolution was high enough to provide the detail needed to show all the features.
Methods
As was hinted to in the first exercise, we needed to normalize the data so that we could put the data into ArcGIS. The original table seen in exercise 1 gave us a grid with letters on one side and numbers on the other, which could not be put into ArcGIS. I needed to first create a table with x, y and z-values in separate columns. As the table below shows (fig. 1) I took the letters and made them into numbers (A=1, B=2, etc.) and left the y-values as previously numbered. I took each letter and stacked the data on top of each other. So I changed all the A column into 1 and then started on the B column right below as you can see in the figure below which was changed to 2. This was repeated for all of the numbers.
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| Part of the finalized table with the corrected x and y values, ready for ArcGIS. (fig. 1) |
Next, I put the table into ArcGIS and created points based on the x and y columns in the table. I did this by importing sheet1 which is the table within the excel file that I created and just dragging and dropping it into the table of contents. I then right clicked on the table once it was in ArcGIS and selected Display the XY Data. I then saved the resulting points as a feature class by exporting it into my geodatabase. The output was a grid of data points and the image below shows what the points looked like in ArcGIS(fig. 2).
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| Original layout of the data points imported from the table in ArcGIS. (fig. 2) |
Once I had the data points in I was able to run some 3D-analysis tools. I ran five different tools that are used to extrapolate x, y and z-values into a surface. They all use different methods to create a surface between the given points based on the other points around them. I ran a IDW (fig. 3), a Natural Neighbor (fig. 4), a Kriging (fig. 5), a Spline (fig. 6) and a TIN (fig. 7). Each of the tools had the same window open with places to input a Input point feature, z value field, and location of the Output raster. There were some more optional inputs that I didn't use in this exercise.
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| IDW model (fig. 3) |
The first interpolation was the IDW shown above (fig. 3). I input the point feature I made above, set the-z value to the z-column in the points feature and that was the resulting output raster. IDW is an interpolation raster surface that is made from points using an inverse distance weighted (IDW) technique. "[IDW] interpolation determines cell values using a linearly weighted combination of a set of sample points. The weight is a function of inverse distance. The surface being interpolated should be that of a locationally dependent variable" (1).
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| Natural Neighbors model (fig. 4) |
The next interpolation technique was Natural Neighbors (fig. 4). I did the same as the last interpolation where I set the point feature as the input and z-value to the z-column in the points feature class. The resulting raster above shows a surface using the natural neighbors technique. "The algorithm...finds the closest subset of input samples to a query point and applies weight to them based on proportionate areas to interpolate a value (Sibson 1981)" (1).
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| Kriging model (fig. 5) |
The third interpolation technique I used is the Kringing technique (fig. 5). I again input the point feature and set the z-value as the z-column in the point feature. The raster above is the output of the interpolation using kriging. "Kriging is an advanced geostatistical procedure that generates an estimated surface from a scattered set of points with z-values...[Kriging] involves an interactive investigation of the spatial behavior of the phenomenon represented by the z-values before you select the best estimation method for generating the output surface" (1).
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| Spline model (fig. 6) |
The last interpolation technique I used was the Spline model (fig. 6). I again input the point feature and set the z-value as the z-column in the point feature. The raster above is the output of the interpolation using spline. "Spline...uses an interpolation method that estimates values using a mathematical function that minimizes overall surface curvature, resulting in a smooth surface that passes exactly through the input points" (1).
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| TIN model (fig. 7) |
After the interpolation methods we also wanted to add another model which was a Triangulated Irregular Network or TIN (fig. 7). This is done by creating triangulated surfaces between the points provided. I input the point feature and z-values in the same way as the interpolations above.
My group discussed the different models and we concluded that the best one was the Spline model because of its smooth look and close resemblance to the actual terrain. However, we noticed that in the bottom left third of the terrain there was a canyon that was not properly shown in any the models. We needed to resurvey this area and create new points and model as well.
We went back out to the sandbox and resurveyed the area that covered the canyon not properly displayed and surveyed points every 5 centimeters instead of every 10. The image below shows the new points added to the previous points which should fix the error in the original terrain model. The portion we resurveyed was 50 centimeters by 60 centimeters and was 30 centimeters right of the origin (bottom, left-hand corner) and 50 centimeters above. This area covered the valley that was not modeled correctly and once we resurveyed would give us the detail we needed.
I then was able to add the newly surveyed points to the table and import that table back into ArcGIS just like I did above with the original table. I then used the table to Display XY Data the same as before and exported the new points into a points feature class in my geodatabase. The resulting points are shown below (fig. 8).
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Revised layout of points after the resurveying of the terrain.
Providing a more detailed representation of the terrain. (fig. 8) |
I was then able to create a new Spline 3D-analysis model of the terrain using the updated data and giving me a more accurate model. I just input the new points feature class and set the z-value as the new z-column in the points feature. Below is a new terrain model including the new points (fig. 9). We chose to do it using the Spline tool because that gave us the best model for our terrain with the original data.
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| Finalized Spline model using the revised data points. (fig. 9) |
The image below shows the area that was changed in the revised version of the data survey and shows the valley that was supposed to be there (fig. 10). The revised version has much better detail and gets rid of the "land bridge" between the ridges that shouldn't be there. The resurveyed points get rid of any misrepresentation caused by the low resolution of the first survey and a more detailed and accurate model in the end.
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Close up of the area that was revised.
The resurvey giving us a more detailed representation of the valley (fig. 10) |
Discussion
The data we collected was very good and closely matched the real life terrain we created in the sandbox. We were able to transfer the data from our grid drawing of the sandbox into the (x, y, z) points with relative ease. We just need to stack the x-values on top of each other and change the x-values from letters into numbers. This wasn't a difficult problem to fix.
Once we made the models and found the errors that needed to be addressed we were able to quickly resurvey and create more accurate data with great results. We didn't have to resurvey any other part of the terrain though, which was lucky. The area we did resurvey was a quick procedure and fixing this problem was also not too hard.
Selecting the Spline model was a easy pick due to its smoothness, especially compared to the other interpolation methods. Our terrain was very smooth however and other group's terrains might have needed something with more sharp or jagged looking surfaces.
Conclusion
In this exercise we were able to use our critical thinking skills and problem solving skills to fix any problems that we had such as the formatting of the data, resurveying the terrain and the choosing of the best interpolation technique.
This exercise showed us the importance of resolution and the needed for higher resolution when dealing with complex features. It is important to know what resolution is needed for certain projects once we enter our careers because you don't want to waste money on high resolution when it's not needed and you don't want to have inaccurate data with too low of resolution.
This exercise was a fun challenge that utilized our ability to circumvent problems and improve our accuracy in data collection and model building as well as knowing what resolution to use in certain places.
Sources
1. Interpolation technique descriptions provided by ArcGIS 10.2.2 Help.
