If you trace over a bitmap image, carefully placing a vector or curve over lines in the bitmap, and then zoom in, the relative position of the vector or curve will change relative to the bitmap line. The reason is complex, but has to do with the fact that bitmaps are made of pixels, and they display at different sizes as you zoom in and out. The lines and curves are always displayed the same size.

If you trace part of an image at one zoom scale and another part at a different zoom scale, the two parts may not be positioned correctly relative to each other.

If you import several bitmaps, say partial scans of the same original drawing, they may not be imported at the same scale.

Here is a partial solution to these problems.

To Import Multiple Bitmaps

1. Open an empty file.

2. Pick a layer for an import size reference. Draw a single horizontal line on the layer that spans the view from left to right.

3. Lock the reference layer. Nothing else can go on this layer. Be absolutely certain that nothing else ever gets into this layer!

5. Use "View/Fit To Window" (CTRL W) to resize the view so the reference line spans the window.

6. Import the first bitmap on a separate layer. DO NOT ZOOM OUT TO VIEW THE ENTIRE BITMAP!

7. Import the next bitmap on another layer, etc. Repeat until all bitmaps are imported.

8. If you have to zoom in/out you can always disable all layers except the reference and zoom to fit window again to get back to the original import zoom level.

This procedure will allow you to import multiple bitmaps all at the same scale.


To Trace Bitmaps

1. After all bitmaps have been imported zoom in to a level where you can see enough detail suitable for tracing.

2. Pick a layer for a trace size reference. Draw a single horizontal line on the layer that spans the view from left to right.

3. Lock this tracing reference layer. Nothing else can go on this layer. Be absolutely certain that nothing else ever gets into this layer!

4. Proceed with tracing the bitmap(s).

5. If at any time you want to zoom in/out to look at the drawing, do it.

6. After changing the level of zoom, before you resume tracing disable all layers except the trace size reference layer. Then use "View/Fit to Window" to get you back to the proper zoom level for tracing.

This procedure will ensure that all your tracings are at the same exact scale, and are positioned correctly relative to each other.