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Chapter 6. Getting Information From Spatial Objects

6.1 Text and Numeric Output

All spatial objects used by LandSerf are fully georeferenced. This allows rasters and vectors to be co-registered when overlaying one on the other. To display this information, select the relevant objects from the thumbnail view and select either Info->Summary Info or the button button. This displays the title, bounds, map projection information, notes and colour table associated with the spatial object (see figure below). This information may be changed by selecting the relevant item from the Edit menu.

Spatial object summary information Spatial object summary information.

Univariate statistics for a raster can be calculated by selecting the Info->Statistical summary menu item. This will calculate measures of average, dispersion, spatial autocorrelation (local roughness) and fractal dimension. You also have the option of displaying a variogram that indicates how roughness changes with scale.

Raster statistical summary Log-log variogram Raster statistical summary and log-log variogram.

More detailed information about raster and vector attributes can be found by querying the spatial object interactively with the mouse. To do this, place LandSerf in Query Mode by either selecting the Info->Query Map menu item or by toggling the button button. By moving the mouse over the main LandSerf display, location and attribute associated with the current mouse position will be displayed at the bottom of the main window. A permanent record of these query results are displayed in the LandSerf console and recorded in the LandSerf log file (see Chapter 1 - Introduction). If a spatial object is associated with an attribute table (see Chapter 3 - Creating, Editing and Transforming Data for more details), the value displayed will be determined by the active attribute selected from that table. This allows textual as well numerical values to be displayed. If a secondary raster or primary vector is selected, clicking on a location while in query mode will display both these attributes while highlighting the selected vector object on screen.

6.2 Graphical Output

6.2.1 Surface Profiles

To query more than one raster cell value at a time, cross-sectional profiles can be displayed by selecting either the Info->Profile menu item or the button button. By clicking somewhere on the main raster display and dragging the mouse to another location, linear cross-sections are displayed in the profile window. The labels along the X-axis give the distance from the first point in the profile in ground units. The number of sample points along the profile can be controlled by the slider at the bottom of the profile window.

Interactive elevation profile output Interactive elevation profile output.

If you have a vector map selected as a primary vector in addition to a raster elevation model, you have the option of viewing the profile along the selected vector. To do this, select the Use vector option then click on the chosen vector in the main LandSerf window. This vector will be highlighted on screen and the profile updated accordingly.

6.2.2 Surface Parameters

It is possible to query various surface parameter values interactively using the mouse. To do this, make sure a DEM is displayed and select the Info->Multi-scale query... from the Info menu. This opens a dialogue box asking for the window scale and parameter type to be selected (see Chapter 7 - Performing Analysis on Surfaces for more details on these parameters).

Multi-scale query options Multi-scale query options.

By dragging the mouse over the main display, output similar to that shown below is produced. The graph shows the value of the selected parameter (slope, curvature, feature type etc.) on the vertical axis, and the spatial extent over which the parameter was measured on the horizontal axis. Thus the curve produced shows how the given parameter varies with scale.

Interactive multi-scale query output with negative curve accumulation Interactive multi-scale query output with negative curve accumulation.

The numeric values at the bottom of the window represent the location of the point being queried and either the mean and standard deviation of the queried parameter over all scales, or its minimum and maximum vales, depending on what was selected in the Query Options window. If aspect is selected as the parameter to query, the circular mean and standard deviation are displayed. If categorical parameters such as feature type are selected, the mode and entropy are displayed. The higher the standard deviation or entropy, the greater the scale dependency of the measure.

Interactive multi-scale output of feature type and aspect queries Interactive multi-scale output of feature type (left) and aspect (right) queries.

Curves are updated dynamically as the mouse is dragged over the surface. Old curves can be left on the display by moving the slider towards the Trails end. Leaving old curves on the graph allows the spatial variation in scale dependency to be shown as a query area is moved over a surface. They way in which old query curves are displayed can be controlled by toggling between the button and button buttons. Positive accumulation means that as successive lines are drawn over the same location, their representation becomes darker. Negative accumulation means that as curves are replaced with newer ones, they become progressively lighter. Pressing the button button will clear the current graph display.

6.2.3 Scatterplots

To compare the values of two rasters, select the Info->Scatterplot menu item or the button button. This will plot the currently selected primary raster as the independent variable on the horizontal axis, and the secondary surface as the dependent variable on the Y-axis axis. The number of samples taken from the two rasters can be controlled with the slider.

Scatterplot output comparing elevation and slope Scatterplot output comparing elevation (X) and slope (Y)

6.2.4 Frequency Histograms

To examine the frequency distribution of a raster, select either the Info->Histogram menu item or the button button. This plots a frequency histogram of the primary raster using its colour table. The class width of the histogram can be controlled via the slider at the bottom of the graph. Moving the slider to the right increases the width of each class and therefore the total number of classes. Surfaces that contain large flat areas such as lakes or sea can produce histograms dominated by the elevation of the flat region. Such areas can be removed from analysis by ticking the Ignore values at box and supplying an appropriate value.

Frequency histogram of elevation surface Frequency histogram of elevation surface

Many DEMs exhibit artifacts of the original contour lines that were interpolated to create the surface model. This can sometimes be detected by examining the frequency distribution of elevation values. For example, a DEM derived from 10m contour lines may show higher frequencies of 10m, 20m, 30m... elevations than 5m, 15m, 25m... elevations. This effect can be visualised by plotting the frequency histogram not of the elevation values directly, but the modulus (remainder) to the base of the suspected contour interval. To do this, select the Info->Hammock plot menu item. DEMs that show this effect will result in a 'U' shaped hammock plot. DEMs that do not show this effect will typically show a rectangular distribution.

Note that large flat areas (e.g. lakes or sea) will significantly affect the hammock distribution, so as with the frequency histogram, a user-defined value can be ignored in the calculation. The modulus value used for calculation of the plot defaults to 10, but can altered by moving the slider.

Hammock plot of elevation surface showing bias towards multiples of 10 Hammock plot of elevation surface showing bias towards multiples of 10