Data quality•GIS is a garbage magnifier (Garbage in / garbage out)•Most failed GIS projects are due to poor planning and poor data quality•Whenever you work with spatial data (or any data for that matter) you will deal with some sort of error due to the many steps involved in creating spatial data. •Spatial data is just an abstraction of what is really there. Because of this abstraction, we can expect error due to:i.How we conceptualize the data in the first placeii.How we collect the dataiii.How we present the data
Data qualityThe following criteria are used when analyzing the internal data quality:1. Accuracy:•is the degree to which information on a map or in a digital database matches actual/ true values. •The discrepancy between the encoded and the actual value of a particular attribute for a given entity is defined as an “error”.•Types of accuracyi.Positional accuracy-a term used for the degree by which positions on a map or in a GIS database are recorded correctly with respect to their true location on the earth’s surface. For example, accurately measured GPS coordinates will fall into the wrong administrative boundaries due to wrong positions of the boundaries in the base map.
Data qualityi.Positional accuracy-•It is divided into two different categories:•Absolute accuracy: refers to the actual X,Y coordinates of a geographic object. If one knows the correct position of the geographic object, they can compare the differences with the position represented in the geographic database. •Relative accuracy:refers to the displacement of two or more points on a map (in both the distance and angle), compared to the displacement of those same points in the real world. •The figures on the below show two different maps of the Cornell campus and the City of Ithaca. The left map, a USGS quadrangle, has an absolute accuracy of around 40 feet. That is, the coordinates for a building on the quadsheet are probably within 40 feet of their real world coordinates. The right map, a photogrammetrically derived map of the same area has an absolute accuracy of about 2.5 feet.
Data qualityii.Temporal accuracy- refers to the agreement between encoded and ‘actual’ temporal co-ordinates of an entity. Temporal coordinates are often only implicit in geographical data, e.g., a time stamp indicating that the entity was valid at a precise time. iii.Thematic accuracy- refers to the correctness of the descriptive information linked to the spatial data (attribute) values encoded in a database.iv.Logical accuracyonly pertains to correct representation/ the integrity of relationships among geographic features. For instance, a river in one GIS database layer must connect to a bridge in another layer. Another example, a river stored in a hydrologic database that defines the boundary between two administrative units should coincide with the units in a neighboring map.