Need an easier way to compare past times and current times in same map display

[jeffreyameyer]

What's your idea for a cool feature that would help you use OHM better. A primary use case for a historical map is a fast and easy way to compare what was going on somewhere across at least 2 time periods.

For example, what did [current map pane] look like in the [past, future]?

This is often accomplished with:

1. A transparency to show two time periods (e.g. Map Warper)

2. A vertical slider of some sort that you can move back and forth to reveal or hide underlying maps (e.g. Old Maps Online)

3. Toggling between 2 different views and hoping you remember what was on the prior view (e.g. OpenHistoricalMap

The problem with the third option – our current method of flipping between map views – is that it triggers a refresh of the entire map pane. This means the user's attempt to remember what the prior view (say the current time) looks like is interrupted by blue sea, the loading of tiles, and filtering/styling/rendering of data. Slower connections aggravate this issue.

Also, this capability might be applied to comparing the vector data to any old rectified historical map, as opposed to another OSM-based layer. There's a hint of this from iD underlays...

[danrademacher]

I am struck by the fact that the truly ideal solution would be that history is being made at every moment, "yesterday" is fully present in OHM, and so one compares times simply by using the timeslider and setting it to yesterday.

But that dream is a long way off. And while the use case here is to compare OHM default layer with OSM standard, as a way to compare "then" and "now", the detailed request is to compare OHM vector layer to any arbitrary raster layer.

For side-by-side, there's an existing Leaflet control for that, updated last year though with an open PR for a critical issue and otherwise a bit long in tooth, https://github.com/QuantStack/leaflet-splitmap.

This does all make me wonder how far away we might be from MapLibre+vector tiles showing up in upstream OSM. If we build a feature like this in ways that are deeply embedded in Leaflet

[jeffreyameyer]

This ticket is 2 feature requests, so let's split them out:

• a time comparison slider
• raster map "underlays": now, #820

I'd say that #820 is higher priority than this ticket, but open for discussion.

[1ec5]

I would definitely limit the scope of this feature request to comparing one OHM style against the same OHM style but across different dates. Otherwise, we’d only invite users to make unflattering comparisons to OSM or other sources based on “completeness”, totally ignoring the fact that we have all the dates and they have none of them.

We’ve had a date-to-date feature on the wiki for over a year, using the maplibre-gl-compare plugin. The Leaflet plugin above sounds analogous. The wiki can fit the map comparison into a relatively tiny box because it doesn’t need a time slider – the two dates are predetermined based on what we want to showcase. For a more general-purpose feature, we’d probably need to streamline the time slider’s design: #648.