R3 — Read the license before building a product on open data; CC0 ≠ CC BY-SA ≠ ODbL

Rule: Before building a derivative product on open data, read the license. CC0 (public domain) requires nothing; CC BY / ODC-By require attribution; CC BY-SA and ODbL require share-alike on derivative databases. US federal works are generally public domain; OpenStreetMap is ODbL.

Why: A share-alike obligation on a downstream database is not a deal-breaker but it changes the build economics. Attribution requirements are easy to satisfy but easy to forget. The penalty for getting it wrong scales with how public the derivative product is. See OpenStreetMap uses the Open Database License (ODbL) — attribution + share-alike on derivative databases; "produced works" (rendered maps) can be licensed freely and US federal government works are generally public domain — OPEN Government Data Act (P.L. 115-435) + 17 U.S.C. §105; agencies encouraged to use CC0.

How to apply:

• License audit happens at project-scoping time, not at launch.
• The use-case matters: internal-only use is generally exempt from ODbL share-alike; published derivative databases are not.
• When in doubt, default to the more permissive (CC0 / public-domain) source.