Prioritizing resurvey tiles when the budget won't cover the whole country
Every maintenance cycle starts with the same arithmetic problem. The quad index has a few thousand tiles on a five-year or seven-year rotation, the field season has a fixed number of crew-days, and the two numbers have never once lined up. Some agencies solve this by stretching the cycle: tiles due in year four slip to year six, then year eight, and nobody updates the schedule to say so. Others solve it by brute force, sending crews out in strict rotation order regardless of what's changed on the ground. Neither gets you a defensible answer when someone in the budget office asks why a subdivision built in 2019 still shows pasture.
Rotation order and ground truth are different lists. The tiles due for a scheduled visit this year are rarely the same tiles where the ground actually moved since the last edition, and the gap between those two lists is where most of a mapping agency's resurvey budget quietly leaks.
Rank by evidence of change, not years since last visit
A rotation schedule treats every tile as equally likely to be wrong, which was never true. A tile over a stable forest reserve and a tile over a growth corridor on the edge of a metro area don't age at the same rate, even if both were surveyed on the same date five years ago. Ranking tiles by calendar age alone sends crews to re-walk ground that hasn't changed and leaves genuinely stale tiles waiting for their scheduled turn.
What separates the tiles worth a field visit this season from the ones that can wait another year is evidence: new rooftops, realigned road segments, a reservoir drawn down below its mapped shoreline, a quarry that's doubled in footprint. That kind of evidence used to mean someone paging through recent orthophoto against the current basemap tile by tile, which is its own time sink. Comparing continuous high-resolution imagery against the basemap layer and flagging where the two disagree turns that into a sortable list instead of a manual review.
Build the worklist, then spend the budget top-down
Once you have tiles ranked by how much they've diverged from the current edition, the budget conversation changes shape. Instead of asking "can we afford to cover the state this year," you're asking "how far down this ranked list does this year's crew-days get us." That's a question a budget office can work with directly, because it ties dollars to a specific, auditable list of tiles rather than a percentage of a rotation schedule.
It also changes what you tell field crews before they drive out. A worklist built from change scores comes with a reason attached to each tile: new construction, a shifted river channel, a road network that no longer matches the centerline file. Crews show up knowing what they're checking for instead of doing a blind resurvey and hoping they notice what's different.
The tiles that don't make the cut aren't dropped from the picture. They stay on the list, ranked lower, and get re-checked against new imagery as it comes in rather than waiting for their scheduled year to roll around again. A tile that looked stable in March and gets a new access road cut through it in July moves up the list on its own.
The ground doesn't change on a fixed interval, so a worklist built on one will always drift from what's true on the ground. National Change Alerts keeps a ranked list of tiles current between field seasons, built from continuous imagery compared against your basemap, with the tiles at the top representing the biggest measured change since the last edition.
If next year's resurvey budget is going to fall short of full coverage again, it's worth seeing what a change-ranked worklist would have told you to do differently this year.