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2.3 Analytical detection limits 
Managing chemical compound analysis within a database generates several problems, among them coding the detection 
limits is an example. On analytical sheets the detection limits are commonly preceded by the "<" sign ; if using this 
symbolic, we work with strings and not either with numbers. As a chemical analysis references only positive values, it 
is possible to replace by convention the "«" by "-" sign. If this artifice allows the data query, a new problem arises for 
the computation or graphic data representation. Even if we lost information, the most interesting way is to take into 
account the absolute value of results. Nevertheless in all cases, there is a loss of information, since we can not define a 
specific value ; indeed for a same chemical compound, we find sometimes different detection limits according to the 
laboratory analysis method, the dilution, etc. 
3 DATA REFERENCING 
3.1 Generalities 
It is known that the specificity of spatial information systems is the notion of geo-reference / location. The location data 
may be obtained from digitized maps or, to have a better reliability, from a DGPS (Differential Global Positioning 
System) survey. Anyway, it is necessary and essential to define a cartographic projection : in our study, we use metric 
Lambert I coordinates (north of France). 
3.2 Spatial references reliability 
3.2.1 Initial documents. It is very difficult to obtain informations on industria] sites history. Most data come from 
studies perform on already brown fields. Thus, there is often a lack of information, and when data are available the 
precision of referencing is usually low. In our cases, three kinds of maps were used: 
-  Sketch-plans; 
- Some photocopies of CAO plans without information on reduction/enlargement scale factor. 
Apart from these imprecisions, the coordinates are relative without true geographic reference; hence the necessity to use 
a geo-referencing tool to get calibration data. 
- Manual transfer of objects between two geo-referenced maps has generated a series of 
imprecision and incertitude causing finally a translation with the real positioning. 
For accurate geographic reference, such documents can not be used without treatment and require the use of a reliable 
tool for geographic referencing. 
3.2.2 Field works. Except when realized by surveyors, positioning on site often involves the use of approximate 
methods as mentioned by Chaussier ef al. (1981). These methods are not able to provided a resolution as good as metric 
or sub-metric that is required for large scale applications. Thereby, the positioning by DGPS has been used here. 
3.3 DGPS application in the study 
Quickly, DGPS performances are highlighted with regard to initial map reliability. The use of this system has been 
optimized with the realization of several referencing surveys. DGPS has been used to plot point objects (piezometers, 
cores, etc.), linear objects (limits of site, roads, etc.) and polygonal objects (factory buildings, decanting zones, etc.). . 
Moreover, even if they are brown fields, these sites know an activity especially with the temporary waste dumping of 
soils. These soil transfers generate some modifications of the site topography and occupation. It is the reason why 
another DGPS use has been to re-locate cores from an old investigation. 
Later, a new survey at a larger scale has been realized to obtain reference marks on an aerial photograph. 
3.4 Geo-referencing results 
To compare the DGPS data precision and a geo-referenced and ortho-rectified aerial view (precision 1 pixel # 50 cm) 
several tests have been realized. Figure 1 shows the overlay of the two kind of data: 
- Building location (A) is hidden exactly on the aerial photograph since only the building shadow is 
visible; 
- A A difference is highlighted between limits of site (B) and the road network; 
- Roads (C) overlie road network. If the road legend is more wide, the network view should be 
totally hidden. 
- The waste dump (D) location is also exactly defined. 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000. 1157