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/.../src/roscomm.cpp:222:85: error: ‘IMU’ is not a member of ‘sensor_msgs’                     
   typedef sync_policies::ApproximateTime<sensor_msgs::NavSatFix, std_msgs::Float64, sensor_msgs::IMU> NavSyncPolicy;
                                                                                     ^~~~~~~~~~~
/.../src/roscomm.cpp:222:85: error: ‘IMU’ is not a member of ‘sensor_msgs’

This sounds like you forgot to include the sensor_msgs/Imu.h.

Note: the message name is Imu, not IMU.

another note: time synchronisation is only possible with message types that have a header: std_msgs/Float64 doesn't have one, so it cannot be used.

/.../src/roscomm.cpp:222:85: error: ‘IMU’ is not a member of ‘sensor_msgs’                     
   typedef sync_policies::ApproximateTime<sensor_msgs::NavSatFix, std_msgs::Float64, sensor_msgs::IMU> NavSyncPolicy;
                                                                                     ^~~~~~~~~~~
/.../src/roscomm.cpp:222:85: error: ‘IMU’ is not a member of ‘sensor_msgs’

This sounds like you forgot to include the sensor_msgs/Imu.h. header.

Note: the message name is Imu, not IMU.

another note: time synchronisation is only possible with message types that have a header: std_msgs/Float64 doesn't have one, so it cannot be used.

/.../src/roscomm.cpp:222:85: error: ‘IMU’ is not a member of ‘sensor_msgs’                     
   typedef sync_policies::ApproximateTime<sensor_msgs::NavSatFix, std_msgs::Float64, sensor_msgs::IMU> NavSyncPolicy;
                                                                                     ^~~~~~~~~~~
/.../src/roscomm.cpp:222:85: error: ‘IMU’ is not a member of ‘sensor_msgs’

This sounds like you forgot to include the sensor_msgs/Imu.h header.

Note: the message name is Imu, not IMU.

another note: time synchronisation is only possible with message types that have a header: std_msgs/Float64 doesn't have one, so it cannot be used.

/.../src/roscomm.cpp:222:85: error: ‘IMU’ is not a member of ‘sensor_msgs’                     
   typedef sync_policies::ApproximateTime<sensor_msgs::NavSatFix, std_msgs::Float64, sensor_msgs::IMU> NavSyncPolicy;
                                                                                     ^~~~~~~~~~~
/.../src/roscomm.cpp:222:85: error: ‘IMU’ is not a member of ‘sensor_msgs’

This sounds like Note: the message name is Imu, not IMU.

If you've fixed that and still get the same error, it's likely you forgot to include the sensor_msgs/Imu.h header.

Note: the message name is Imu, not IMU.

another note: time synchronisation is only possible with message types that have a header: std_msgs/Float64 doesn't have one, so it cannot be used.

Edit:

Is this an error with synchronizer.h I can't fix, was it caused by a minor error in my code?

seeing as these packages have been around for at least 10 years, I would always assume your code is at fault -- at first.

However, no code is faultless, so if you've assured that you're doing the right thing, then suspecting message_filters starts to make sense.