So why is it rare for stereo imagery to be sold as a "product"?
Personally I think there are a few reasons for this. These include:
- There's no practical standard for storing photogrammetry metadata (e.g. image exterior orientation parameters). Although there are efforts underway, photogrammetric metadata is typically stored in proprietary formats. This inhibits the ability of the data to move from system-to-system. For example, if the imagery was triangulated in System X and then provided to a person using System Y, they invariably have to go through some pain and suffering to ingest the data (running an import job at a minimum).
- Airborne flight operations are expensive. Data providers are typically contracted to fly specific jobs for clients such as regional authorities and so forth (here is an example). Times may be changing, but I haven't seen too many companies out there flying stock imagery and then reselling it. This is partly tied to the point above - without a common system for storing photogrammetric metadata, it is difficult for data vendors to deliver a one-size-fits-all solution.
- The workflow is perceived as being difficult. For example, you need specialized and expensive stereo viewing hardware, domain knowledge in photogrammetry, etc. It all depends on the application, but if you're not performing stereo work all day long as your primary job, then it is still possible to get high-accuracy results working in split-screen or anaglyph mode. As for the domain knowledge, once the imagery is triangulated, then derived products such as the ones above are very easy to create - although they can be time-consuming depending on the project size.