Date: 201912
Local time of day: 0001-0600
Location: ZSE.ARTCC, WA
Flight conditions: VMC
Light: Daylight
Aircraft: Small Aircraft
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Personal
Flight phase: Cruise
Airspace: Class E ZSE
ATC / advisory: Center ZSE
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Human-Machine Interface; Distraction; Situational Awareness
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance
Contributing factors: Airspace Structure; Human Factors; Weather
Primary problem: Airspace Structure

Narrative: Aircraft X was handed off at 10;000 feet going westbound. The aircraft's route display line was south of an airway that has an MIA (Minimum IFR Altitude) of 13;000 feet; a GPS alt of 10;000 feet; and a MOCA of 9;000 feet. The route display line also appeared to just miss an MIA of 10;500 feet that begins approximately 40 miles inside my airspace and only lasts for approximately 10 to 15 miles. The initial MIA along the route for the aircraft; in those first 40 miles; is 9;000 feet. The airway runs through the extreme southern portion of the MIA and is somewhat of a visual cue for the MIA boundary. Upon initially taking radar of Aircraft X I ran out a route line and it appeared to just miss the higher MIA. As the aircraft got closer to the MIA and the MSAW alert activated; it still appeared that the aircraft would remain outside the 10;500 feet MIA.After issuing a few clearances and doing coordination with other aircraft and facilities; I looked at Aircraft X again; and ranged in on the radar as it appeared the aircraft had drifted just north of his route line and the upper leading edge of the target was beginning to enter the 10;500 feet MIA. I issued a 15 degree turn to the aircraft to attempt to keep it out of the higher MIA. When the turn did not appear to be working I assigned a 500 feet climb to get the aircraft to the 10;500 feet MIA. At this point I had ranged back out to work some of my other traffic. After a couple more control instructions to other aircraft; I ranged back in again and it appeared that the center of the target line had crept ever so slightly north of the MIA line; into the 10;500 feet MIA. I notified my supervisor that I thought the aircraft might have gotten into the higher MIA. Once Aircraft X was level at 10;500 feet; I cleared him direct to their fix. A few minutes later I issued Aircraft X his descent clearance to 9;000 feet as prescribed in our Letter of Agreement with the TRACON.Personally; in the future; if able; I will just climb the aircraft to the MIA even if the route line appears to miss it; especially if the route line is extremely close; and it is a General Aviation aircraft. The other option would have been to just vector the aircraft to the north a bit; and then cleared it to join the airway both the GPS altitude and MOCA altitude would have been usable and 'safe;' and allowed the aircraft to stay at 10;000 feet. Having checked the winds afterward; I noticed that they were light out of the south at 10;000 feet which was slightly different than the quartering winds above 18;000 feet and likely contributed to the aircraft drifting slightly north of its route line. I also do believe; however; that the line for that MIA could be reviewed and likely moved north a bit to line up with the airway; or even possibly just north of the airway. In my opinion; having essentially 4 different 'usable' restrictive altitudes along that stretch of airspace is more confusing than helpful. I don't know exactly what causes the MIA to be 10;500 feet in that area; but I assume its further north where the majority of that MIA exists.Also; if the airway GPS altitude is 10;000 feet and the MOCA is 9;000 feet; and less than a mile or so south of that the MIA is 9;000 feet; it seems very counter intuitive that the MIA boundary for the 10;500 feet MIA needs to extend south past the airway. I don't know if it is possible for something like that to be reviewed; but I do believe it warrants consideration. Finally; it would be a great improvement if there could be some sort of warning for MIA's along an aircraft's route; similar to those we get for Special Use Airspace. Due to a lot of changing and 'non-standard' MIA's and a large number of awkwardly shaped MIA's in our airspace; we get a lot of 'non-event' MSAW warnings as aircraft often fly quite close to; or even through MIA's along their route. As a result; I believe there is some complacency with regard to the MSAW warning. A secondary indicator on the display would be helpful to identify aircraft that are definitely filed; or whose route line actually penetrates MIA's that are not safe for their altitude.




Synopsis: ZSE Center Controller reported that an aircraft deviated from its assigned vector into a higher MIA area.