Error terrain alt variance with sensor

hi guys i have pixhawk cube with arducopter 3.5.7 , i add ultrasonic sensor max1200 analog downward .
i want to use the terrain following and i want to keep alt of 2 m .but when i start working with it i get the error of the “error terrain alt variance” and in some places i lose my alt and again keep up .my speed was 300cm/s .
i use this page to for terrain following :
how should i solve this problem