Abstract A prototype wireline tool which includes a downhole inertial platform and a surface computer to spatially map a well is described. The hardware consists of a single-gimballed inertial platform with accelerometers and gyros to obtain three-axis motion information. The gyroscope and accelerometer outputs are transmitted to a computer at the surface which calculates probe attitude relative to north, east, and vertical. Double integration of the accelerometer data provides the position information. A conventional 7-conductor wireline is used for the system data transmission. System accuracy is enhanced by advances made in the computer software which processes the data received from the tool. The software uses statistical sampling estimation to obtain optimal estimates of the system errors. Measurement errors are determined by periodically stopping the tool during the logging procedure and observing the indicated velocity measurements. This procedure, known as Kalman filtering, results in increased accuracy of the data. Present mapping systems have an X-Y-Z location accuracy of 100 to 200 feet for a typical well depth of 10,000 feet. Test results show that the new system is accurate to about 1 foot per 1000 feet of well depth. Unlike conventional systems, the inertial navigator does not require any sort of projection of the cable length (which may not be accurately known). Also, this system provides continuous data throughout the wellbore and logging speeds on the order of 10 ft/sec appear possible. The hardware and software associated with this mapping system are described and the recent field test results are reported.