Methods: Raster-based time series analysis
H. Mitasova, M. Overton, E. Hardin, M.O. Kurum
Time series analysis uses multitemporal elevation DSMs to elucidate and quantify trends in terrain dynamics using th following measures:
- Standard deviation from mean elevation is a measure of how dynamic a region is. Low standard deviations indicate stable regions, high values indicate dynamic regions, and very high values can often be used to indicate human influence like the construction of new homes.
- The minimum value shows the most minimum elevation of each location over the entire time period regardless of what time it was at its minimum. It is important to realize that the minimum surface never actually existed at any time but is the composite of the lowest lying areas out of every map. The volume of earth below the minimum surface represents the earth that has never moved throughout the entire time period; it is called the core surface for that reason. Areas with a low lying core surface may be at risk due to storm surge etc.
- The maximum surface is similar to the minimum surface except that it is the maximum elevation at each location. This is called the outer envelope. There was never any terrain above the outer envelope at any given time. The volume between the outer envelope and the core is called the dynamic layer because all terrain evolution occurred within this layer.
- The range in z values is the difference between the envelope and core surfaces. It is the thickness of the dynamic layer, and it can be used as a measure of moving sand. For example, structures that stand on a thick dynamic layer may be at risk because while the earth is there now, at one time the elevation there was significantly lower and it may be likely to return to that state.
- Maps can also be created in which the z-value actually shows the time at which the terrain elevation was at its minimum and maximum. This map would show when and where the core and envelope surfaces actually existed.
- Slope indicates the rate and direction of change over the study period. Negative slopes are show areas of erosion and positive slopes show areas of accretion. Note: zero slope does not mean that the area is not changing, it just means that the net rate of change was zero.
- Coefficient of determination indicates how linear the slope is and ranges from zero to unity. High values for coefficient of determination indicate highly linear change while low values indicate sporadically dynamic behavior or can also indicate a change in the direction of slope, i.e., change from accretion to erosion or vise versa.