Analysis for quality control in database input for making GIS


M. D. Joshi
Sardar Vallabhbhai Patel Institute of Technology, Vasad, Mailing Address: Sardar Vallabhbhai Patel Institute, of Technology (SVIT) At & Po. Vasad - 388 306, Dist: Anand , Gujarat
Tel: 02692 74766 , 74 357, Fax No. : 02692 74540
E-mail - maheshduttjoshi@yahoo.com

R. Sivakumar
Department of Science and Technology, Mehrauli Road, New Delhi 110029


Testing of data and errors
The vector data was tested for accuracy when compared with the source data. The quality indicator given in terms of the scale were considered for testing, specially in grading into numbers, ranging from 0 to 9 scale. This provides an opportunity to get an idea about the relative accuracy of the data. In this case a vector data (the source was considered to be the accurate one).

Above factors were tried in a sequence. Some of the basic examinations were necessary to be carried out on the data, which are as follows: Lineage was the basic factor considered for giving quality indicators.

1. The vector data was tested for accuracy for all the well defined details and found to be 90% accurate within an error of 0.5 mm and 100% within an error of 1.00 mm.
2. Attribute accuracy was tested based on polygon overlay, and it was found that 95% of the polygons tested are accurate.
3. Positional accuracy and completeness were tested by super-imposing the raster and vector data in the digitization stage itself. And found to be satisfactory.
4. The attribute accuracy was also taken care while giving quality indicator.
5. Logical consistency was checked on the database generated by performing topological tests that were possible in the MGE environment. Topologically clean data extraction was one of the main aims of the study.

This was achieved by ensuring that all the chains intersect at nodes and remains consistent around the polygons. Also, inner rings embed consistently in closing the polygon. This is a rigorous testing and took considerable time to ensure that the data is topologically clean in small polygons.

Errors analysis
The accuracy of data in a GIS is prone to errors from many sources. Right from the stage of aerial photography till the data is burdened with creeping errors. Considering that the maps are derivatives of photogrammetry, the final accuracy of a map may be expressed by law of propagation of variances;

The error was analyzed at various stages in terms of those cropped up at placing the points, called the point errors. Various points were compared and found to have errors. Similarly, it was observed that errors existed while making lines as well as areas. These errors were also compared.

The errors were noted at the photogrammetric mapping stages in terms of difference between map position and photo positions. Similarly, between positions of points as obtained from DTM and photo points etc. However, taking all the combined effect of errors it was noted that these are errors of systemic in nature as well as random in nature and thereby most of the errors compensate for each other except very few which remain with the features in the GIS. On analyzing the total errors, it was observed that the errors are within tolerable limits.

Conclusion
From above study, it was observed that the laid down quality control factors as indicated in the US Spatial data Standards can be used easily and a suitable method can be devised to check the adherence to the standards. In the proposed method a suitable sequence was adopted considering the original source data as an accurate data whereas Raster and Vector data as erroneous data due to digitization and other inherited errors. Although the source data itself may be erroneous by certain amount which needs to be given due consideration. However, in view of the map standards laid down by USGS, the map accuracy could be considered fairly good for accepting the position of ground points as references for such comparison. In view of this fact, the present results obtained from the studies appear to be satisfactory within their own limitation of data used.

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