A Measurement While Drilling (MWD) tool is a combined magnetic steering and surveying instrument run near the bottom of the BHA. It accurately measures azimuth and inclination as well as toolface.
Measurement While Drilling (MWD) has an advantage over a steering tool in that no wireline is required inside the drillpipe since an electric power supply is provided. It is more versatile than other survey instruments - certain MWD tools can provide readings from additional sensors like Gamma Ray (for maintaining correlation and for picking coring or casing point markers) or formation evaluation logging sensors.
Surveys should always be taken after making a connection in order to reduce the chances of stuck pipe. Although the pipe should remain stationary when the survey is being taken, it should be reciprocated during transmission of the data to surface.