Monday, November 30, 2015

Types of errors in GIS data, their elimination and accuracies

Maps created by GIS have deficiencies.  These deficiencies occur due to “Errors” that may have taken place at different stages of GIS implementation. Flaws in data are referred to as errors. An error is the physical difference between the real world and the GIS output. Errors may be single, definable departures from reality, or maybe persistent widespread deviations throughout a whole database. Errors reduce the accuracy of the map generated. Using well defined and controlled procedures errors can be avoided.
There are two types of errors in GIS:
        I.      Source Errors:
                 They are the errors that are present in “Source Data” that is given to the GIS. They occur      before the actual implementation of GIS:

                                            a.      Instrumental inaccuracies:
                                               ● Satellite/ air photo/ GPS/ surveying (spatial).
                                               ● Inaccuracies in attribute measuring instruments.
                                            b.      Human Processing:
                                               ● Misinterpretation (e.g. photos), spatial and attribute.
                                               ● Effects of scale change and generalization.
                                               ● Effects of classification (nominal / ordinal / interval).
                                            c.      Actual Changes:
                                               ● Gradual 'natural' changes: river courses, glacier recession.
                                               ● Catastrophic change: fires, floods, landslides.
                                               ● Seasonal and daily changes: lake/sea/ river levels.
                                               ● Man-made: urban development, new roads.
                                               ● Attribute change: forest growth (height etc.), discontinued trail /roads,                 road surfacing.

        II.      Processing Errors:
                  They are the errors that occur during the processing of the data i.e. during the                       implementation of GIS.

                   a.     Input:
                       ● Digitizing: human error, the width of a line, spikes, knots, also entering attribute                                                      data.
                       ● Dangling nodes (connected to only one arc): permissible in arc themes (river                    headwaters etc.).
                       ● Pseudo-nodes (connected to one or two arcs) - permissible in island arcs, and where    attributes change,
                            e.g. road becomes paved from dirt or vice versa.
                       ● Projection input error.

        b.     Manipulation:
                ● Interpolation of point data into lines and surfaces.
                ● Overlay of layers, digitized separately, e.g. soils and vegetation.
                ● The compounding effects of processing and analysis of multiple. layers: for example, if        two layers each have correctness of 90%, the accuracy of the resulting overlay is                                   around 81%
                ● Density of observations.
                ● Inappropriate or inadequate inputs for models.

         c.     Output:
                 ● Scale changes - detail and scale bars.
                 ● Color palettes: intended colors don't match from screen to Printer.

In addition to the above, errors may also be introduced during data transfer and conversion. These types of errors, may lead to loss of data.
It is a misconception that data from a GIS is of a higher quality. The quality of information from a GIS depends upon the quality  of the data used as input to the GIS.

Precision indicates how closely several positions fall in relation to each other. Precision is the recorded level of detail of the data.

Accuracy is a measure of the closeness of one or more positions to a position that is known and defined in terms of an absolute reference system. Accuracy is the extent to which an estimated data value approaches its true value (Aronoff, 1989).

Bias in GIS data is the systematic variation of data from reality. Bias is a consistent error throughout a dataset. Bias in GIS data may be caused due to human or technical sources.

ERROR, PRECISION, ACCURACY and BIAS affect the quality of individual datasets.

GIS error elimination
The simplest method of checking for data errors in GIS is by visual inspection. Comparison of data in GIS format with the original map reveals major errors. Double digitising is an error checking method used by large companies. This involves digitising the same map twice and comparing the two copies to identify inconsistencies. This is a costly and time consuming method of error checking. Statistical methods can be used to pinpoint potential errors.