  We consider the density field $f(x)$ generated by a volume source $\mu(y)$ in
$D$ which is a domain in $\R^3$. For two disjoint segments $\gamma, \Gamma_1$
on a straight line in $\R^3 \setminus \ooo{D}$, we establish a conditional
stability estimate of H\"older type in determining $f$ on $\Gamma_1$ by data
$f$ on $\gamma$. This is a theoretical background for real-use solutions for
the determination of air dose rates of radioactive substance at the human
height level by high-altitude data. The proof of the stability estimate is
based on the harmonic extension and the stability for line unique continuation
of a harmonic function.
