Getting Started

Using the Comparison Slider

The comparison slider is the heart of ChangingMaps. It lets you see two moments in time side by side and drag between them to reveal how a place has transformed. Understanding how to read the slider will help you spot patterns and changes you might otherwise miss.

When you select two years to compare, the slider divides your screen into two halves. The left side shows the Before year—the older imagery. The right side shows the After year—the newer imagery. A vertical divider runs between them, and you can drag it left and right to reveal more of either image. As you drag, you're essentially peeling back time, watching how buildings, vegetation, roads, and land use have changed in that exact spot.

Pay close attention to the date labels at the top of the map—they tell you exactly which years you're comparing. This matters because it frames what you're looking at. A comparison between 2006 and 2025 spans nearly two decades of change; a comparison between 2018 and 2025 might show more recent development or recovery. The years are your context.

As you drag the slider, look for patterns. New buildings and housing developments often appear as sudden clusters of rooflines or cleared land. Vegetation loss shows up as darker, denser areas becoming lighter and more open. Road widening or new streets create visible lines of change. Suburban expansion spreads outward from existing towns. Parks and open spaces can shrink or grow. These aren't subtle shifts—when you compare imagery years apart, the physical transformation of a place becomes undeniable.

Small details often reveal the most interesting stories.

Don't just look for the obvious changes. Zoom in and pan the map to focus on specific areas—a single street, a neighborhood, a waterway. At street level, you'll notice subtle shifts: individual trees planted or removed, footpaths added, a field converted to houses, a carpark resurfaced. These smaller details are where the real story of place change lives. A tree-lined street that has lost its canopy tells a different story than one that has gained it. A wetland drained for development is visible in the imagery. A school or community facility appearing where there was open land marks a moment in a town's growth.

Sometimes the imagery may appear slightly misaligned between years—a building edge might not line up perfectly, or a road might seem to shift slightly. This is a known limitation of historical datasets called georeferencing error. Different imagery sources, different satellite positions, and different processing methods across decades can introduce small spatial shifts. It's not a mistake in ChangingMaps; it's inherent to how historical aerial imagery is archived and aligned. Once you know to expect it, you can work around it and still see the larger patterns of change.

Take your time with the slider. Move it slowly. Pause and look. Drag it back and forth to compare the same area multiple times. Let your eye pick up on changes that might not be obvious on a first pass. The before-and-after comparison is most powerful when you're patient and curious—when you're genuinely trying to understand how a place became what it is today.

If you want to dive deeper into what the changes mean—what environmental shifts, infrastructure changes, or development patterns you're seeing—each location includes cards that break down these categories. But the slider itself is your window into the raw visual evidence. The imagery doesn't lie. It just shows you what was there and what's there now. The story is in the comparison.