# i am using insfilterNonholonomic filter and i wanted to ask how to conver my gps speed to velocity in local NED coordinates

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ahmed mohammed on 28 Oct 2022
Commented: Junhui on 1 Mar 2024
i am using insfilterNonholonomic filter and i wanted to ask how to conver my gps speed to velocity in local NED coordinates my gps data are "lla , speed ,course, horizontal position accuracy". and how to find velocity accuracy ?

Brian Fanous on 31 Oct 2022
Assuming your device is traveling on the ground (no Z direction velocity, Z position == 0) and we are working in NED....
Your speed is the vector magnitude of your x and y velocity. Your course is the direction of the speed.
speed = sqrt(velx*velx + vely*vely)
course = atan2(vely, velx)
Your velocity accuracy is a function of your GPS device. Perhaps look at the device specification. The tune() method can help you determine the optimal value if you have a ground truth trajectory.
Junhui on 1 Mar 2024
Hi there: I am also using insfilterNonholonomic filter to predict system trajectory. But what if I need to account for altitude change. How would that affect the formula to compute gps velocity (assuming system altitude varies only a little bit as compared to latitude or longitude of system)? Thanks...

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