MANIPULATION AND TRANSFORMATION OF SPATIAL DATA
The maintenance and transformation of spatial data concerns the ability to input, manipulate, and transform data once it has been created. While many different interpretations exist with respect to what constitutes these capabilities some specific functions can be identified. These are reviewed below.
Coordinate thinning involves the weeding or reduction of coordinate pairs, e.g. X and Y, from arcs. This function is often required when data has been captured with too many vertices for the linear features. This can result in redundant data and large data volumes. The weeding of coordinates is required to reduce this redundancy.
The thinning of coordinates is also required in the map generalization process of linear simplification. Linear simplification is one component of generalization that is required when data from one scale, e.g. 1:20,000, is to be used and integrated with data from another scale, e.g. 1:100,000. Coordinate thinning is often done on features such as contours, hydrography, and forest stand boundaries.
This function is concerned with the registering of a data layer to a common coordinate scheme. This usually involves registering selected data layers to a standard data layer already registered. The term rubber sheeting is often used to describe this function. Rubber sheeting involves stretching one data layer to meet another based on predefined control points of known locations. Two other functions may be categorized under geometric transformations. These involve warping a data layer stored in one data model, either raster or vector, to another data layer stored in the opposite data model. For example, often classified satellite imagery may require warping to fit an existing forest inventory layer, or a poor quality vector layer may require warping to match a more accurate raster layer.
Map Projection Transformations
This functionality concerns the transformation of data in geographic coordinates for an existing map projection to another map projection. Most GIS software requires that data layers must be in the same map projection for analysis. Accordingly, if data is acquired in a different projection than the other data layers it must be transformed. Typically 20 or more different map projections are supported in a GIS software offering.
Conflation - Sliver Removal
Conflation is formally defined as the procedure of reconciling the positions of corresponding features in different data layers. More commonly this is referred to as sliver removal. Often two layers that contain the same feature, e.g. soils and forest stands both with a specific lake, do not have exactly the same boundaries for that feature, e.g. the lake. This may be caused by a lack of coordination or data prioritization during digitizing or by a number of different manipulation and analysis techniques. When the two layers are combined, e.g. normally in polygon overlay, they will not match precisely and small sliver polygons will be created. Conflation is concerned with the process for removing these slivers and reconciling the common boundary.
There are several approaches for sliver removal. Perhaps the most common is allowing the user to define a priority for data layers in combination with a tolerance value. Considering the soils and forest stand example the user could define a layer that takes precedence, e.g. forest stands, and a size tolerance for slivers. After polygon overlay if a polygon is below the size tolerance it is classified a sliver. To reconcile the situation the arcs of the data layer that has higher priority will be retained and the arcs of the other data layer will be deleted. Another approach is to simply divide the sliver down the centre and collapse the arcs making up the boundary. The important point is that all GIS software must have the capability to resolve slivers. Remember that it is generally much less expensive to reconcile maps manually in the map preparation and digitizing stage than afterwards.
Edge matching is simply the procedure to adjust the position of features that extend across typical map sheet boundaries. Theoretically data from adjacent map sheets should meet precisely at map edges. However, in practice this rarely occurs. Misalignment of features can be caused by several factors including digitizing error, paper shrinkage of source maps, and errors in the original mapping. Edge matching always requires some interactive editing. Accordingly, GIS software differs considerably in the degree of automation provided.
Interactive Graphic Editing
Interactive graphic editing functions involve the addition, deletion, moving, and changing of the geographic position of features. Editing should be possible at any time. Most graphic editing occurs during the data compilation phase of any project. Remember typically 60 to 70 % of the time required to complete any project involves data compilation. Accordingly, the level of sophistication and ease of use of this capability is vitally important and should be rated highly by those evaluating GIS software. Many of the editing that is undertaken involves the cleaning up of topological errors identified earlier. The capability to snap to existing elements, e.g. nodes and arcs, is critical.
The functionality of graphic editing does not differ greatly across GIS software offerings. However, the user interface and ease of use of the editing functions usually does. Editing within a GIS software package should be as easy as using a CAD system. A cumbersome or incomplete graphic editing capability will lead to much frustration by the users of the software.