Introduction
This lab served as a follow up to the previous field exercise, in which a group of students constructed a sample terrain in a roughly one square meter sand box. Afterwards, each group of students was tasked with sampling and surveying the terrain using a variety of sampling techniques. In the previous lab, a hybrid systematic-stratified sampling technique was used to gather 212 points from across the plot and record their respective x, y, and z data in cm. In the original plot, the x and y "0" values were arbitrarily set to one corner, while the z "0" value was set to the height of the grid overlayed on the terrain, with the point recorded above the grid having a positive z-value (height), and the points below the grid having a negative z-value. In this lab, the data would be normalized in order to be interpolated using a variety of interpolation techniques. Data normalization refers to organizing any collected data logically based on fields organized into a graph or table. However, a list of 212 data-points showing their x, y, and z coordinates holds little value, as it is difficult to visualize the original terrain using just the raw organized data. The interpolation procedure would generate a number of digital 3D models of the original terrain, allowing for simple and effective visualization of the data.
Methods
First, the data was normalized into a series of x, y, and z fields in an Excel document. The data was set a numeric, with the number of decimal values being set for each field in order to prevent computing error for when the data would be imported into ArcGIS. Afterwards, the data was added to a ArcMap viewer by using the 'add XY data' command found under the 'File' menu. Each field of the data was matched to the corresponding x, y, and z values required by the function, and the data was added to the map. In order for the data to be used in a functional form, the X,Y data was exported as its own shapefile, which was added to the map document. The data-points were then color coded to visualize their z values in relation to one another (Figure 1).
In this form, the data could be properly utilized for interpolation methods. Five would be the primary focus of this lab: IDW, Natural Neighbors, Kriging, Spline, and TIN interpolation.
IDW interpolation uses an inverse distance weighting to generate values for points and location not originally sampled. In other words, it uses the surrounding points to generate an average based on how far away the surrounding points are. Due to the nature of this method, ridges and valleys are lost if they were not part of the original survey. However, this survey works well if the original sample was sufficiently dense and recorded most of the ridge and valley features. This method however does maintain the integrity of each point originally recorded. This means that the generated 3D model will show a bump-like pattern across the model, where each bump shows where one of the original survey points was record. This is both helpful and non-helpful. While it preserves the original data, it creates an unnatural looking pattern across the generated model of the terrain.
Natural Neighbors is an interpolation method which calculates a points's z value based on the weighted value of the surrounding points. By weighting each of the surrounding points, this creates a terrain model that is far smoother than a simpler nearest neighbor interpolation, which using the surrounding values without providing appropriate weight to each value.
Kriging is an interpolation technique that makes a mathematical assumption that there is a spatial correlation between sample points that can be reflected in the distance and direction between points. This method generates a model with a degree of predictional accuracy at the cost of adding directional and distance bias in the model. In the end, the model may end up smoother than the original terrain.
Spline interpolation is a method that forces the model to generate a surface with passes through the recorded points while bending as little as possible between the points. This model effectively measures gently varying features like slope and elevation very well, at the cost of losing the natural curvature of the original terrain.
TIN interpolation, or triangular irregular networks, generates a model of the surface by using a series of triangles which make up the surface between all sampled point. This, like IDW, preserves the original values of the sampled points. Unlike IDW, it does not have the problem of generating a bump covered surface. Instead, terrain features are largely maintained at the cost of generating a simple looking model without the natural curves and bends of the original terrain.
Regardless of which method used, they all largely involved similar steps. Each model was generated using the corresponding 3D analysis tool found in ArcMap and by inputting the original data-point feature class into the function. Each resulting interpolation model could be visually in 3D using ArcScene and by adjusting the settings to correctly show vertical exaggeration in each of the models. From here, each model was oriented with the original x,y orientation of the original survey. Each interpolation model was then exported as both a JPEG and a VRML file. Each JPEG file of each interpolation method was arranged with a photograph of the original sandbox terrain (Figures 2-6). The orientation was set the way it was to both make comparisons to the collected data-points and original terrain simpler and more effective. The original terrain, along with minimum and maximum z-values for both the terrain and generated models, provided the scale for each interpolation. Scale and orientation are critical for these exports, as without them the 3D generated models lose meaning and value to real-world context. Without scale and orientation, they cannot be connected back to the real world phenomenon used to generate the model.
Results
Of the interpolation methods used, the Spline interpolation method seems to have generated the most accurate representation of the original terrain. In the IDW model, the characteristic bumps can be see typically generated by the method (Figure 2).
While preserving the original minimum and maximum z-values, this has generated a model which is hindered by a distracting phenomenon which was not present in the original terrain. The Natural Neighbors interpolation method generated perhaps the second closest digital representation of the original terrain. Most of the original terrain features are preserved, along with the corresponding x and y values (Figure 3).
However, the model appears more jagged than the original terrain did. The Kriging model, in contrast, looks far too smooth when compared to the original terrain . Do to the Kriging technique focusing on distance and directional relation between points, many of the important surface features have been lost. The ridge and valley of the original terrain have been largely flattened and smoothed out. Even the maximum and minimum z-values have been greatly reduced in regards to the original terrain (Figure 4).
Of all the interpolation methods, spline seems to have generated the most representative digital terrain. Its strikes a balance between the smoothness of the Kriging method while maintaining the jagged ridge and valley best represented by the Natural Neighbors methods (Figure 5).
In doing so, the maximum z-values, and perhaps all recorded z-values, have been distorted. The maximum z-value is greater than the original, while the minimum z-value is lower than the original. Terrain shape has been maintained by sacrificing the integrity of the original recorded values. TIN perhaps best represents the original points survey. All of the original survey points maintain their original x, y, and z values. However, this model is lest looking the roughest because of this. All non-recorded data is represented by triangles which bridge the gap between data-points (Figure 6).
This leaves the model looking primitive in comparison to other interpolation methods, which used some for of mathematics to determine z-values for points not originally recorded. In the end, all of these methods combine to create an accurate representation of the terrain originally surveyed, and provide the stepping off point for the digital mapping of the survey data. If this survey were to be performed again, more points would be collected from the bottom half of the survey area, and less from the top half. The number of data-points for and surrounding the ridge were perhaps too few, and there were too many points recorded from the plain of the original terrain, creating too large of a focus on a featureless area.
Conclusion
This survey provides what would be an effective staring off point for any large scale survey. Using this small scale terrain survey, both a viable survey technique can be constructed for a larger area and a interpolation method may be chosen to correctly display what the large scale survey intends to focus on. The only real difference this survey has from a large scale one is the size of the survey and the relative ease this survey was conducted, in contrast with a full-scale one. This sandbox terrain survey was conducted with minimal effort using simple tools and relatively few people (3) in a short span of time (2 hours). In contrast, a full scale county survey may take days or weeks pf hard work, with often a grid based survey being impossible due to both time constraints and a sometimes inability to reach the desired sample-point location by convenient means of travel. Interpolation methods, in addition, can be used for other forms of data. Water tables, soil types, and precipitation are just a few of the forms of data that can be measured and represented by survey and interpolation methods.
Sources
ArcGIS Help,
In ArcGIS Resources. Retrieved 2/14/2017, from
http://resources.arcgis.com/en/help/
Hupy, J. (2017).
Sandbox Survey part 2: Visualizing and refining your terrain survey.. Eau Claire, Wisconsin.