Digital Mapping Techniques '01 -- Workshop Proceedings
U.S. Geological Survey Open-File Report 01-223

From Conventional Airphotos, to 2.5D Maps and Digital Mosaic Terrain Reconstructions: A Simple but Complete Digital Technique Used at GSC-Québec, Canada

By Serge J. Paradis¹, Marco Boutin², Ruth Boivin³, and Hugo Larocque^2,4

1Geological Survey of Canada Québec Geoscience Centre 880 Ch. Ste-Foy, CP 7500 Ste-Foy, Québec, Canada, G1V 4C7 Telephone: (418) 654 2556 Fax: (418) 654-2615 e-mail: sparadis@nrcan.gc.ca 2INRS-Géoressources Québec Geoscience Centre 880 Ch. Ste-Foy, CP 7500 Ste-Foy, Québec, Canada, G1V 4C7 e-mail: ess-inrs-mboutin@x1.nrcan.gc.ca

3Natural Resources of Canada ESS-Info 880 Ch. Ste-Foy, CP 7500 Ste-Foy, Québec, Canada, G1V 4C7 e-mail: ruboivin@rncan.gc.ca 4Present address: 1157 rue De Liesse Masson-Angers, Québec, Canada,J8M 1H7 e-mail: hugo.larocque@statcan.ca

INTRODUCTION

In 1997, the Québec Geoscience Centre acquired a Digital Video Plotter (DVP), which is a photogrammetric software package. It allowed upgrading the methodology for transferring data with an accurate and relatively fast method, from multi-scale conventional airphotos to multi-scale topographic bases. With time, we developed multidisciplinary applications for this software.

THE OLD DAYS

Figure 1. Mapograph reflector projector.

Furthermore, the only way to minimize radial distortion was by manually displacing the airphotos on the support located under the glass compartment of the projector. Also, with the mapograph used at GSC-Québec, if the scale of the airphotos was more than 2.5 times that of the topographic base, the transfer step had to be repeated. This double manipulation was time consuming and added the possibility of introducing errors to the final product.

NEW TECHNIQUE

Figure 2. Digital Video Plotter with digitizing table.

Thus, it is possible to transfer information from airphotos that have a significant scaling difference compared to the topographic base (eg: airphotos at 1:40 000 scale on a topographic base at 1:250 000 scale) in a single operation (Paradis and Parent, in press a,b; Paradis et al. 2000). Since the airphotos are scanned at 400 dpi resolution (image size 8.5 MB); it is possible to use them while conserving their original scale (23 cm x 23 cm format) or any other scale following a reduction or a zoom on the cathodic screen. Therefore, the airphotos can be reported on the topographic base, whatever its scale, without adding errrors in the transfer procedure.

Digital videorestitution is a videogrammetric technique that allows 2D or 3D visualisation of digital images or graphics on a cathodic screen. To optimize the use of the 2D technique, which is favored at GSC-Québec, two very important steps are needed to recreate the geometric situation that prevailed when the original photos were taken. This topic has been previously discussed in detail by Paradis and Boutin (2000a) so we will only succinctly mention them for the benefit of the present reader.

1. Internal orientation: consists of defining the focal distance of the camera when the photo was taken and the registration of the exact distance between the fiducial marks (located in the four corners of the photo, represented by a circle or a cross depending on the type of camera).

2. Absolute orientation: as mentioned above, consist of estimating transformation parameters for the coordinates between the stereoscopic model and the numerical topographic base. These steps require special attention, because they control the precision of displacement, following x and y axes, of captured elements from the airphotos to the compilation on the digital topographic base. For a map at 1:20 000 scale and air photos at 1:15 000 scale (Paradis et al. 1999), the degree of precision in the transfer of information is on the order of 1 meter or less. Alternate positioning techniques such as global positioning systems or electronic total stations allow us to increase this precision. For comparison, the width of a line traced with a rapidograph pen (point 00) on an airphoto at 1:15 000 scale corresponds in the field to a band of 7.75 meters wide.

THE PROCEDURE AT GSC-QUÉBEC

The procedure, step by step, is as follows:

1. the geologist finishes his field work and starts his photo-interpretation, mapping all geologic or geomorphologic features that have to be integrated to the final map compilation; these informations are directly inscribed with a special pen (00 rapidograph with black ink) on the original airphotos;

2. the airphotos are scanned at 400 dpi., creating derived tiff files of 8.5-10 MB (we are presently using a scanning digitizer Sharp JX-610 with a resolution of 600 dpi; almost any modern regular scanning digitizer could do the job);

3. each individual photo has to be oriented individually using the DVP orientation module (internal and absolute orientation);

4. each individual photo is orthorectified, so that they can be perfectly superposed on to the numerical topographic base; this is done using the DVP rectification module;

5. individually rectified airphotos are merged together (groups of 16-20) into a large mosaic creating one image / derived file of 50-60 MB using the DVP mosaic module;

6. a decision has to be made at this point: will the end product be a digital terrain reconstruction or a 2.5D map compilation;

7. to obtain the digital terrain reconstruction the mosaic must be draped on a TIN (Triangular Irregular Network) that is created using the Geoterrain Microstation module;

8. to produce the final 2.5D map compilation, the mosaic is imported into the Descartes Microstation module where all the linework, polygons, linear structures or any geological or geomorphological element that are not points, are digitized;

9. the same mosaic is then transferred in the DVP digitizing module where all the other elements are digitized; the merging of the two mosaic files (Descartes, DVP) is done within the DVP using DXF files; importing in Descartes is also possible;

10. when the digitization is complete, a DXF file is created and imported into ArcInfo where the data are separated in three files: a) the geological contact file used to build polygons; b) the geological linear feature file (ex: moraines, eskers) and c) the geological point feature file (ex: outcrop, striae); the three files are then corrected, standardized and edited;

11. following this editing, the different map covers are prepared (ex: boundary, hydrology, topography, roads etc.);

12. the legend is compiled and edited;

13. using ArcInfo and GRID (ArcInfo module) a hydrology file is created and a Digital Terrain Model (DTM) is built using a 2 step AML semi-automated procedure;

14. using GRIDCOMPOSITE (ArcInfo command) we can superpose the geological color map (completed at step 15) on the DTM;

15. map composition AMLs are then prepared including logos, scale, location map, and the 2.5D map then is ready for printing.

CONCLUSION

ACKNOWLEDGMENT

REFERENCES

Paradis, S.J., 2000b, Géologie des formations superficielles, Lac Inconnu, Québec (32 G/13): Commission géologique du Canada, Dossier public 3847, 1 color map, scale 1:50,000.

Paradis, S.J., 2000c, Géologie des formations superficielles, Lac des Orignaux, Québec (32 G/14): Commission géologique du Canada, Dossier public 3898, 1 color map, scale 1:50,000.

Paradis, S.J., and Boutin, M., 2000a, L'utilisation d'un vidéorestituteur numérique comme outil de cartographie détaillée (Saguenay, Québec): Recherche en cours 2000-D13, 8 p., .

Paradis, S.J. and Boutin, M., 2000b, La vidéorestitution numérique, technologie multi-scalaire adaptée aux besoins de la cartographie géoscientifique: IXe congrès de l'Association québécoise pour l'étude du Quaternaire conjointement avec la réunion annuelle du groupe canadien de recherche en géomorphologie, Programme et résumé de l'AQQUA - CGRG Montréal, août 2000, p. 9 (Conférence).

Paradis, S.J. and Parent, M., in press a, Géologie des formations superficielles, Lac Bermen Est, Québec: Commmission géologique du Canada, Carte 1976A, scale 1:125,000.

Paradis, S.J. and Parent, M., in press b, Géologie des formations superficielles, Lac Bermen Ouest, Québec: Commmission géologique du Canada, Carte 1977A, scale 1:125,000.

Paradis, S.J., Parent, M., Boutin, M., Boivin, R. and Larocque, H., 2000, Cartographie détaillée 2.5 D (1:250,000) des formations superficielles de la région de la rivière Koroc (24 I) dans le Grand-Nord québécois dans Séminaire d'information sur la recherche géologique, 2000: Résumés, Ministère des Ressources naturelles du Québec, DV-2000-03, p. 50.

Paradis, S.J., Parent, M., Perret, D. and Bégin, C., 1999, Géologie des formations superficielles, Saint-Fulgence et La Baie, Québec: Commission géologique du Canada, Dossier public 3710, 2 color maps, scale 1:20,000, 22D 07-200-0201 and 22D 07-200-0101.