The count was right and the pins were 400 km away
RuleDerive, do not invent.
In July 2026 a map sandbox answered a simple question, how many wells are in one county, with a number that looked right. Then it drew the pins. They landed about 400 km away, near a large city.
The data was synthetic, and the generator had assigned each record a county name at random. It had also assigned each record a position. Each value was plausible alone. Together they described wells that sat in one county and claimed to be in another. Every county's records covered the same rectangle of the map.
The part that cost the most time was the test. An acceptance check compared the count in the app with the count in the database, and they agreed every time. Both were reading the same invented column, so the check passed while every geographic answer was wrong.
The fix was small. The positions were fine and the labels were wrong, so I threw the labels away and worked out each county from the point, using real county boundaries. A county should never have been a value someone typed in. It is a fact about where the point is.
A person who knows the region sees this kind of error in one glance, and after that they stop trusting the count, the chart and the next demo as well.
How I build demo data now
- If one field depends on another, I compute it. County comes from the point and age comes from the birth date.
- I look up the real totals and ranges before generating anything, then scale to them.
- I give every region a user might ask about some data, unless the real total is so small that most regions truly have none. Then zero is the honest answer.
- I seed the generator so the same input gives the same dataset on every rebuild.
- I validate against something outside the system: geometry, a reference layer or a published figure.