Displaying length of feature on page in print composer?

Question

I'm making a map for a walking trail in QGIS. In the below screenshot, the red line is the trail.

The trail is about 60km in length in total, but on this page of the map, the trail is about 23km long (I eyeballed this, it's probably off by a bit).

The trail is a single, long line feature.

What I would like to do is have the length of the trail on this page of the map be displayed.

I know that I can get the total length of the feature, but I haven't got the faintest idea how to measure the trail on a per-page basis.

Answer 1

If you already have polygon features you are using to demarcate the edges of each map - a grid index or similar - you can just use this to split the line into multiple line features using the Intersect or Clip tools to calculate lengths individually (Vector overlay > Clip OR Intersection. Clip will simply return the original line attributes while Intersection will return the polygon attributes as well with the output feature layer, which may be helpful when referencing to a specific map). Reference this length on each map. The split lines layer can be turned off if you wish to use the full line feature and not have to mess around with multiple symbologies.

If you don't have anything to define map boundaries - from what I understand you're developing multiple maps to follow the route of this trail, so it may be worthwhile implementing some form of automation to make your job easier - you can use the PolyStrip plugin to perform this function (https://plugins.qgis.org/plugins/polystrip/). If you aren't already, you might also want to look into the Atlas functionality of QGIS, which facilitates the creation of a series of maps based on polygon-defined locations (this question Does QGIS feature a tool like ArcGIS Data Driven Pages? and a tutorial https://www.qgistutorials.com/en/docs/automating_map_creation.html)