What’s the difference? Other than an ‘i’.
Ordinance is a law, or rule and regulations, made by a government or municipal authority eg. ‘The draft ordinance is currently under debate’.
Ordnance refers to military supplies, especially weapons and bombs, or to large guns on wheels eg. ‘Do not touch any military ordnance that may be found lying around this area’. The most familiar use is in Ordnance Survey, the national mapping agency for Great Britain, which publishes large and small-scale maps, and which was formed in 1791. The name reflects the original military purpose of the archetype organisation which was the mapping of Scotland in the aftermath of the last Jacobite Rebellion, and the Battle of Culloden in 1746.