You are correct by saying that the dip angles vary greatly depending on the contours that you pick. In contrast to artificial maps designed to teach basic structural geology to students (such as the one you added in your question), natural maps are rarely "nice and friendly" regarding dip calculation.
There are several reasons for this:
- The dip angle may actually vary. In artificial maps, the beds are usually tabular and not folded (unless this is a folding exercise). Natural rocks are rarely exactly planar and tabular, and their thickness also varies. This can result in different dips for the top and the bottom of the bed at the same location.
- Maps are not usually drawn with dip calculations in mind. The contacts are based on the limited exposures available to the mapping geologist. The contacts between rock units may be obscured by vegetation, talus, soil and even houses and roads. Making a map that is dip-calculation friendly is time consuming, if possible at all.
- It also depends on the accuracy of the topographic contours. As you probably know, you have to take the elevation into account in these calculations. Even if the contours are accurate, the topography may be not accurate or not spaced properly. I can testify from my own experience that the topographic map that was available to me of this area:
seems like we are walking on a somewhat planar area without any steep slopes and only several meters of variability in elevation.
So what are your options? Some maps already have dip measurements built into them, thanks to the detailed work of the mapping geologist. If not, some structural maps of the same area may be available from the local geological survey/society. If you insist in making your own calculations, and the bed is rather planar, you can try to use a wider area for calculation (let's say 1 km instead of 100 m). You can try to take many measurements over an area and make a histogram of the measurements, or maybe examine the spatial distribution.