The topographic map below shows part of the boundary between the Ducktrap River watershed and the Megunticook River watershed located in coastal Maine.
The blue, green and red lines all represent the same watershed boundary. The blue line was drawn first when mapping one watershed and the green line was drawn some time later when mapping the adjacent watershed. Variations result from different interpretation of the same map. Both Green and blue lines were drawn based on Delorme Topo USA 7.0 Data Series maps. The Red line was drawn based on USGS 7.5 Minute Series (Topographic) maps.
The three interpretations fall well within 500 feet of each other giving an acceptable level of confidence in the consistency of the mapping technique.
In reality, the above discussion relates only to mapping consistency and not to the actual watershed boundary. Often, field checks are required to determine the actual direction of surface water flow. Man made objects, especially roads, will have far reaching effects on water flow.
A field check of the area mapped above revealed some interesting yet typical deviations from the mapped boundary.
The red pin in the upper left of the map to the right is a GPS generated way point set during a field check of the data. This was at the top of an obvious ridge and it was easy to verify the watershed boundary.
Field checks are more difficult when the terrain is flatter as in the lower right. The bottom pin (Wpt006) represents what field notes refer to a nebulous but likely watershed boundary. The pin directly north (Wpt 007) locates a drainage culvert which diverts water under the road. It was noted that the road acts to block the free flow of surface water downhill at this point and a farmer's berm west of the way point channels water toward the culvert.
The yellow line was drawn to indicate a more likely watershed boundary after field observations were taken into account.