"磁针偏差计" can be translated as "magnetic needle deviation meter" or "compass deviation indicator".
"Magnetic needle deviation meter" directly refers to a device that measures the deviation of a magnetic needle.
"Compass deviation indicator" is also appropriate, especially if the context is related to navigation or compasses, as it clearly indicates the purpose of indicating deviation in a compass.
Both translations are accurate, and the choice may depend on the specific context or technical field.