Maps and GPS - Stitching (sections of) maps

Introduction

Before you start stitching your map together, it is helpful to study the manual of your graphics program. Many programs work with layers and witch requires some routine. Look at the cut and paste function, find out the way layers can be moved mutually, check the "undo" function and the way you can change the program settings.If you are working with scanned map, study the rotation function. It may be necessary to rotate an image under a small angle.It is also important to know the peculiarities (or features) of your program. This includes the maximum size (height and width) of an image, how many "undo" steps you can make, how many files you can open at the same time, etc.

Hands on

First an important warning: save your work frequently. This prevents much frustration. While editing graphic-files can become very large. The chance that your graphic program crashes is not inconceivable.

Apply a structure in to the stitching of your map sections. If you have a specific area in your mind, you may want to start in a corner and proceed row by row or column by column. If you have an area around a specific point in your mind, you may want to start at that point and proceed in a spiral until you have enough coverage.

Use at least two reference points for the stitching process. The first one you use the elements fit the two images together and the second and possibly third one to check the fitting. Sometimes it is necessary to move one image some pixels to get the best fitting on all the reference points.

It is possible that the two sections are mutual rotated. You must rotate the second section to get the best fit. The angle of rotation you can determine by experiment or by calculation. The attached spreadsheet can be helpful.

Finalizing

when all the stitching is done the reduction of the color depth to 256 colors (8 bit) and saving your map in a proper file format are the next steps. In my note "File formats and color depth" I will tell you about some specific aspects. On this place more general indulgences.

If you want to use the your "raw" map (after the stitching) several times it is convenient to store that map "raw" (with 16 million colors and all the "ragged" edges). So you don't lose any information.
The finalizing is the moment to reduce the pixel resolution to for example 254 pixels per inch.
Some online maps (Dutch, British and Swiss) show coordinate grids with exact horizontal and vertical grid lines. In such cases I prefer to "cut up" my "raw"map into "tiles", about 10 x 10 km, 20 x 20 km or 40 x 40 km, depending on the scale and pixel resolution. You can stitch these "tiles" quickly together to compose your "work" map.
When saving your map I recommend to include the country, location and scale of your map.

Stitching Offline-maps

In some cases offline maps can be stitched together. If the map sheets have no borders and the edges are exactly right-angled it is a matter of sliding against each other. If the map sheet has borders, just "cut them off" and slide them against each other (provided that the map part of the sheet is right-angled) .

Right-angled maps aren't always the case. For example, in Canada and United States the width and height are defined in degrees and/or minutes longitude and latitude. Also at certain map projections aren't right-angled (for example, a conical). As a result, the left and right map edge aren't parallel and that the top and bottom edge are slightly bent. Many graphic programs can not deal with this phenomenal. "Pasting" above each other will often not work. Horizontal "pasting" works when rotating the map slightly. In these cases it isn't recommended to stitch more than two or three map sheets together. Be aware: at each rotation is the map become less accurate.