An underground line locator comprising: a receiver configured to receive magnetic field signals from an underground line and to provide raw measurement data from the received magnetic field signals, the raw measurement data indicative of a location of an underground line relative to the receiver; an inertial sensor coupled to the receiver by a rigid member and configured to generate motion data of the underground line locator; a processor coupled to the receiver to receive the raw measurement data and coupled to the inertial sensor to receive motion data, the processor configured to: determine a state of motion from the motion data of the underground line locator, set a responsiveness from the state of the state of motion, the underground line locator being characterized in that the processor is configured to: determine a primary measurement signal from the raw measurement data, including filtering with a band-pass filter with an adjustable bandwidth adjusted according to the responsiveness, determine a secondary measurement signal from the primary measurement signal, including filtering with an adaptive low-pass filter with a low cutoff frequency that is adjusted in accordance with the responsiveness, provide a display signal indicating a measurement signal from the primary measurement signal and the secondary measurement signal; and in that it further comprises a display coupled to the processor, the display configured to show the display signal, wherein the responsiveness is the speed at which the display responds to changes in the raw measurement data. a receiver configured to receive magnetic field signals from an underground line and to provide raw measurement data from the received magnetic field signals, the raw measurement data indicative of a location of an underground line relative to the receiver; an inertial sensor coupled to the receiver by a rigid member and configured to generate motion data of the underground line locator; a processor coupled to the receiver to receive the raw measurement data and coupled to the inertial sensor to receive motion data, the processor configured to: determine a state of motion from the motion data of the underground line locator, set a responsiveness from the state of the state of motion, the underground line locator being characterized in that the processor is configured to: determine a primary measurement signal from the raw measurement data, including filtering with a band-pass filter with an adjustable bandwidth adjusted according to the responsiveness, determine a secondary measurement signal from the primary measurement signal, including filtering with an adaptive low-pass filter with a low cutoff frequency that is adjusted in accordance with the responsiveness, provide a display signal indicating a measurement signal from the primary measurement signal and the secondary measurement signal; and in that it further comprises determine a state of motion from the motion data of the underground line locator, set a responsiveness from the state of the state of motion, the underground line locator being characterized in that the processor is configured to: determine a primary measurement signal from the raw measurement data, including filtering with a band-pass filter with an adjustable bandwidth adjusted according to the responsiveness, determine a secondary measurement signal from the primary measurement signal, including filtering with an adaptive low-pass filter with a low cutoff frequency that is adjusted in accordance with the responsiveness, provide a display signal indicating a measurement signal from the primary measurement signal and the secondary measurement signal; and in that it further comprises determine a primary measurement signal from the raw measurement data, including filtering with a band-pass filter with an adjustable bandwidth adjusted according to the responsiveness, determine a secondary measurement signal from the primary measurement signal, including filtering with an adaptive low-pass filter with a low cutoff frequency that is adjusted in accordance with the responsiveness, provide a display signal indicating a measurement signal from the primary measurement signal and the secondary measurement signal; and in that it further comprises a display coupled to the processor, the display configured to show the display signal, wherein the responsiveness is the speed at which the display responds to changes in the raw measurement data.