The study of physical properties of planetary nebulae (PNe), and , in
particular, the determination of their luminosity function is hampered in our
Galaxy by the poor knowledge of their distances.
This problem can be overcome by going to external galaxies, like the
Magellanic Clouds, where the distance is known with a reasonable accuracy and
where, in first approximation, all PNe can be taken to be located at the same
distance.
Several photographic surveys, either by direct imaging or through
objective-prism, have been devoted in the past to the search of emission-line
objects in the Clouds, but their limited sensitivity did not allow the
detection of the faintest objects even at these nearby distances.
Progress in detector technologies however open the prospect to get complete
samples of such objects in Local Group Galaxies in the near future.
The possibilities of photographic plates can however still be further
exploited in the meantime.
Sky Survey Schmidt plates are ideal tools to measure a large number of
objects spread over a large area like the Magellanic Clouds.
They allow easily a determination of positions with an accuracy better than
0.5
, a significant improvement over available coordinates.
Such accurate coordinates are nowadays indispensable for follow-up work, with
ground-based facilities like multi-object spectrographs or for centering with
space instruments.
They are further necessary for cross-identification with other surveys,
devoted specifically to PNe or more general ones, like the Deep European Near
IR Sky Survey (DENIS).
At the same time, these Schmidt plates provide finding charts still necessary
for detailed study in crowded regions like the Bar, charts often missing in
earlier surveys.
Finally, reasonable magnitudes (to 
accuracy) can be provided in an
homogeneous set for a large sample, allowing a first sight at the luminosity
function.
We have therefore embarked on a program of systematic measure of positions
and magnitudes of the known PNe in both Clouds by digitizing Survey Plates
(blue and red) over the whole Cloud area, with pixels of 10 microns (or
0
67).
This work had to be supplemented by CCD observations, to recover some of the
cataloged, but unidentified PNe and provide photometric zero points.
This first paper describes the methods and first results in the Large Magellanic Cloud. Section 2 describes the main PNe catalogues used as input. We explain the observations and reduction techniques in Sect. 3, while Sect. 4 gives the results. We finally work out, as an example of possibilities opened by these results, the cross-identifications with IRAS data in the last section.