Boi controller reported a BE20 was descended into a higher MVA.

Attributes
ACN	1564106
Time
Date	201807
Local Time Of Day	1201-1800
Place
Locale Reference	BOI.TRACON
State Reference	ID
Aircraft 1
Make Model Name	Super King Air 200
Flight Phase	Descent
Route In Use	Vectors
Flight Plan	IFR
Person 1
Function	Approach Supervisor / CIC
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 16
Events
Anomaly	ATC Issue All Types Deviation - Procedural Published Material / Policy Inflight Event / Encounter CFTT / CFIT

Narrative:

Aircraft X was being vectored for a visual approach to ZZZ; a satellite airport in the bozeman sector. He was level at 13000 [ft]. My airspace boundary map and my MVA map were set at the same intensity so I mistakenly thought he was in a 12000 MVA so I assigned 12000. 30 seconds later I realized that he was still in a 13000 MVA so I reassigned 13000 but he had already started a descent. Instead of climbing back to 13000 (he was about 12500) he called the airport in sight and I cleared him for a visual approach. I am going to change the intensity settings on my MVA map and my airspace boundary map so that the lines don't look the same and it will be clearer what the MVA is.

Original NASA ASRS Text

Title: BOI controller reported a BE20 was descended into a higher MVA.

Narrative: Aircraft X was being vectored for a visual approach to ZZZ; a satellite airport in the Bozeman sector. He was level at 13000 [ft]. My airspace boundary map and my MVA map were set at the same intensity so I mistakenly thought he was in a 12000 MVA so I assigned 12000. 30 seconds later I realized that he was still in a 13000 MVA so I reassigned 13000 but he had already started a descent. Instead of climbing back to 13000 (he was about 12500) he called the airport in sight and I cleared him for a visual approach. I am going to change the intensity settings on my MVA map and my airspace boundary map so that the lines don't look the same and it will be clearer what the MVA is.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.