I'm trying to improve indoor gps accuracy in ios application. Do people apply such filters to gps data in different apps which track your position? I have 10-20 meters error indoors when using raw data from corelocation so will Kalman Filter have any impact on accuracy?
Some developers do, particularly for tracking run/walk distances. However they are not simply filtering the GPS input, they improve accuracy with sensor fusion. As mentioned in the comments, the GPS data from the receiver should already be filtered so adding another filter wont help. However fusing the data from another sensor, an accelerometer to determine if a user is moving or pressure sensor to track altitude changes, etc. can improve accuracy a great deal and an Extended Kalman Filter is a good way to fuse sensor data so if used properly, yes.