Raster Data Overlays
DEM data are forms of raster data
We can make use of these data types for creating continuous spatial layers for overlay queries or presentations of results. The term DEM is specific to representing changes in in elevation (the E in DEM), and other representative layers can be created to illustrate changes in topography (Hillshading and hipsometric tints).
Fig 1: Shaded relief and Hipsometric Tints
DEM derived layers
Topography as an element of your query
If you are performing a query that is based on topography (i.e. the anthropology queries we did in the lab) then you will need to make use of a DEM. Another product that can be created from a DEM (in addition to the slope, aspect and shaded relief we created in the lab) are flow direction indicators. This could be a watershed created by a GIS user such as yourself to verify the location of potential historical streams and creeks against current mapped water bodies.
What does Scott mean by historical streams?
Fig 2: Watersheds
Other derivations, like slope and aspect (the ones derived n the lab), can be implemented into a model as well. Proximity calculations can be used to qualify data areas by combining vector and elevation layers.
Fig3: DEM and Vector Layer Overlay
DEM to DSSM
We can make use of DEM techniques to create continuous data layers for representing Social Science information. In other words we can create Digital Social Science Models (DSSM) – this acronym is specific to this class (I made it up) so any better suggestions would be appreciated.
An example of a continuous system may be expressed as a DSSM of the population distribution throughout B.C based on block face data input. We can take the point data from the block face information and create a DSSM.
Fig 4: DSSM from Stas Can data
Import Block points
DSSM – raster model of population distribution
A 3 dimentional perspective of models can be expressed as well
Fig 5: 3D Perspective of Population distribution of EA boundaries expressed in ranges of area.
Overlay of EA boundaries (colur scheme) on DSSM (gray raster layer)
Non DEM (or DSSM) raster models
Raster layers can be manipulated on a cell by cell bases. Vector overlay procedures like intersecting, unions, erase, identity or update, have a similar home in raster GIS but manipulation of data can be achieved through grouping (classification) within layers and combining layers.
Classification (grouping of data) can be utilized to gather statistical (as well as spatial statistical) information. Basic questions can be answered by totalling pixel counts of aggregated data.
Fig 7 Besa Study area
Fig 8 Classification results of Besa Prophet in the MK
These operations can be performed on any type of data (geo rectified or not). As with vector data resultant layers can be produced from combining layers using mathematical expressions.
An interesting perspective on pixel calssification
Fig 8 Query layers in Quantum GIS:
In this illustration you can see that pixels of similar ranges may be grouped together (this can be soley within a layer or it can be a grouping of pixels in the same geographical location between layers) to provide boundaries of information.
Fig6: Vector and Raster Overlay:
In a vector overlay slivers of data brought in by intersecting data layers must be cleaned and built to gain area and perimeter information. By using a raster model area and perimeter are calculated by summing up the pixels. A draw back is in deciding whether a pixel is within a shape our out (software decisions) for pixel counts.