When something on the map doesn’t match up with the physical landscape, the logical question “are I or the map right?” arises. Unfortunately, in most cases, the map is correct, and you’ve made an error at some point. However, it’s worth knowing that there are some features on maps that shouldn’t be trusted.

The most common features to change over time are constructed features: minor powerlines that no longer exist, gates that have been replaced, property boundaries that have moved, etc. Ways also evolve with time, and can easily change their course over time (i.e. since the geographic data used to make the map was collected). Therefore, if a constructed feature doesn’t match up with where you think you are on the map, don’t necessarily assume that you are wrong and the map is right. Focus on location identification using landscape features rather than just constructed ones, and if neither features match up with where you think you are on the map, then you’re probably not where you think you are.

Physical landscape features are far less likely to be wrong than constructed features. According to the ‘Australian Map and Spatial Data Horizontal Accuracy Standards’, the location accuracy of a map must be stated in terms of a percentage value of its true position. For example, the Sydney Heads 1:25,000 scale topographic map states “Horizontal accuracy is expected to be 90% of the well-defined detail within 12.5 m of its true position”.

Modern maps are made using a combination of on-ground geographical data and data from aerial imagery. The result is fairly reliable representations of the landscape. However, maps are only as accurate as their geographic data they are made with, and the historical notes of the surveyor. Aerial imagery, for example, can interpret wrong landform shapes if the tree line does not match the ground line. Hence topographic maps are susceptible to these sorts of errors, and can include features that are out of date or have changed, or are symbolized and named incorrectly.

BWRS has a current list of known map errors for NSW topographic maps and offers a service to report errors.