Engagement
Below the fold
Also called scrolled content.
Below the fold is content that needs a scroll before it enters the viewport. Whether it gets seen depends on how many people scroll that far.
How it is measured
Any element whose top edge sits lower than the viewport height at load is below the fold. Scroll-depth buckets tell you how many sessions reached it, and a scroll map shows the same fade as colour.
Tarsier does not draw scroll maps. If you need the data, send a custom event when a marker element enters view and read its count against pageviews.
Worked example
A hiking guide places its trail map after 1,400 px of intro text. On phones that puts the map 1,900 px down. The editor's scroll data shows 54 percent of readers reach the halfway point and 21 percent reach the map.
The map moves up to 700 px. Map interactions rise from 190 to 540 a week, and the intro text is untouched.
How it differs
Below the fold is the part of a page that needs a scroll. Above the fold is the first screen. Scroll depth counts how far people actually went. Content can still perform below the fold if the page gives people a reason to keep going.
Common errors
Burying the main action down there. Assuming nobody scrolls. Counting a sticky bar as below the fold. Hiding key facts for design reasons. Judging from a desktop screenshot.
In practice
Check scroll depth on your top ten pages. Move anything critical to where at least half of visits reach, and use the lower region for detail and reference material.