I have large maps which require an A3 scanner to copy to PC.
Is there a free programme anywhere I can use to stitch together separate parts which have been scanned on an A4 scanner?
Thanks

Scan them as tiff/raw/some other uncompressed image format and then use a image editor...which ever one that is included with Windows these days (scanner/image utility) to do the stitching.

When working with items larger than your scanner bed, it is a good idea to use some sort of index mark on the image...

I haven't tried it...

http://hugin.sourceforge.net/

Just about any free decent graphics program (http://www.makeuseof.com/tag/10-free-image-editing-programs-for-your-pc/) will be able to do what you want but they vary greatly in complexity and even the programs designed specifically for "stitching" are not necessarily the easiest to use.

I would try PhotoFiltre first because it has one of the easier to use interfaces. The Gimp will do just about anything but it has a big learing curve.

Basically with, say PhotoFiltre, first open one file and read its properties from the File Menu. Note the size in pixels. Write down the values and double the horiz or vertical value depending on which way you are "stitching". Then choose new file from the file menu and enter the new values as the size of the new image.

If you are short on RAM these manoeuvres may take some time with A3 size images. You will need to have the original scanning very "square" for the best results obviously and bearing in mind that any free rotation of anything not a multiple of 90 degrees will normally lose a considerable amount of quality.

Go back to the original picture and choose copy then highlight the empty image and choose paste. Drag the pasted selection to the relevant corner. Open the other half, choose copy, go back to the incomplete file and choose paste and carefully drag it into position. Save the new image.

Sounds easy but can be tricky enough. Practice to make perfect. You might want to start with a new file that is a bit larger all round than necessary and after you have saved the stitched file you can go back and crop and resize it to your heart's content.

PS Seen mjc's post now. Hugin (windows installer (http://panospace.wordpress.com/downloads/) unless you want to and can compile with C++) is very complex and almost beta but by all means give it a whirl. It's probably going to be of more use in creating panoramas from digital photographs (what it appears to be designed for) than to stitch together two identically sized images imported from a scanner.