International Meteor Organization
Hints for measuring X, Y coordinates from Atlas Brno
For reporting positional data it is important that the scale of the charts remains unchanged. This means the distance between the thin crosses has to be 70 mm. The origin of the X, Y coordinates is located in the bottom left corner of the chart (X-axis to the right, Y-axis upwards). The reference lines are the inner ones, not the thick lines. Photocopiers often tend to change the original scale. Therefore, use the originals you bought whenever you are going to make copies. Otherwise you merely add to the errors. If the scale error exceeds 3 mm over the whole length of the chart (i.e. if the distance between the thin reference lines differs by more than 3 mm from 280 mm for the short side and/or 350 mm for the long one) you should use another device.
In order to minimize the influence of the remaining scale errors, the coordinates for a given meteor must refer to the same coordinate lines. A ruler of at least 35 cm length is recommended.