Paper maps are issued in a certain scale. The scale is the ratio between a unit of measurement on the map and the same number of units of measurement in reality. The scale of 1:50,000 means that 1 cm on the map corresponds to 50,000 cm (= 500 m) in reality. The primary unit of measure of electronic images is pixel (or dot). Between these pixels and the scale of a map is no direct relation. You must create this relation yourself. From my own experience I know that 50 to 100 pixel per centimeter guaranties good results.
Scanning maps yourself
The resolution of your own scanned map is expressed in pixels per Inch (PPI), where one inch is exactly 2.54 cm. With some number crunching 50 pixels per cm equals 127 PPI and 100 pixels per cm equals 254 PPI. Good values for self scanned maps. If your scanner doesn't supports these values choose 150 or 300 PPI.
Already scanned maps
If the resolution of a scanned map is unknown, it may be helpful to consult the properties of the graphics file. Possible is the scanning resolution included in the "Details" tab. If this is not the case, you have to scrunch (or use the tool) some numbers. Select two points on your map and calculate the distance between them in pixels (possibly using Pythagorean theorem). Calculate the real distance in km between those points (for example, by determining the difference of X and Y coordinates UTM and likewise using the Pythagorean theorem again). By dividing the number of pixels by the number of kilometers you will get the number of pixel per km. Using the following table, you can estimate the original (or a suitable) resolution and scale.
|Pixels per km||Scale|
In the table is the number of pixels per km compared to the scale of the map. The result is the number of pixels per cm. choose basically a scale between 50 and 125 pixel per cm (green areas). If this isn't possible use the yellow areas (between 25 and 50 pixels per cm or between 125 and 175). The formula behind the table is (pixels per km) * scale/100000. For conversion to PPI is the value from the table multiplied by the factor 2.54.
GPS accuracy in relation to the accuracy of the scale
The accuracy of standard GPS sensors is between two and eight meter radial. This depends on the number of satellites the sensor "sees" and any atmospheric disturbances. At a scale of 1:10,000 and a resolution of 100 dot per centimeter (pixel = 1 meter) this means that the GPS cursor may have an deviation of eight pixels, which is visible on your GPS. This is so a semblance accuracy. In my opinion the smallest usable scale (by 100 pixels per cm) is 1:20,000 (pixel = 2 metres) or 1:25,000 (pixel = 2.5 metres).