Clarify difference between thumbnail and overview

[m-mohr]

The STAC Browser implementation assumes the following with regards to the roles overview and thumbnail:

• Thumbnails do not necessarily fit to the extent of the geometry. As such thumbnails are not shown on the map, instead they are just shown as normal images embedded in HTML.
• Overviews usually fit to the extento of the geometry. As such overviews are shown on the map.

Is this understanding of the spec correct? Should we clarify that?

[m-mohr]

From a recent conversation:

If role "thumbnail" is set for a PNG/JPEG image, it doesn't show it on the map. The reason is that thumbnails often don't actually fit to the actual extent of the geometry. Let's say you have a worldwide collection with a thumbnail that only covers a small selected area, you don't want to stretch that incorrectly over the whole globe. You could use the role "overview" if you are sure that placing the image on the map makes sense.
Having that said, that's how STAC Browser behaves. I'm not 100% certain whether the implementation in STAC Browser actually is really how it was meant to be. The spec seems ambiguous about it.

I made a small adjustment to STAC Browser. Follow this:

• If you want to display the JPEG/PNG image on the map use overview and thumbnail as roles.
• If you don't want to display the JPEG/PNG image on the map (often for collections), use only thumbnail as role.