that is exactly what i wana say. i use axMap1.ProjToPixel to convert lon/lat coordinate to pixel coordinate then use axMap1.DrawCircleEx to draw point according pixel coordinate. so reason why i have to get scale index is:
ex: the point move on the way between 2 points, when use axMap1.ProjToPixel its mean the distant of 2 point is measured by the number of pixel but if you zoom map the pixel distant will inscrease => the the position of drawing point (axMap1.DrawCircleEx ) will be change.
PS: thanks for advice about lon/lat from screen, i did it
ex: the point move on the way between 2 points, when use axMap1.ProjToPixel its mean the distant of 2 point is measured by the number of pixel but if you zoom map the pixel distant will inscrease => the the position of drawing point (axMap1.DrawCircleEx ) will be change.
PS: thanks for advice about lon/lat from screen, i did it