Heady, Harold F.; Rader, Lynn (March 1958). "Modifications of the Point Frame". Journal of Range Management. 11 (2): 2. doi:10.2307/3894295. JSTOR3894295.
Heady, Harold F.; Rader, Lynn (March 1958). "Modifications of the Point Frame". Journal of Range Management. 11 (2): 2. doi:10.2307/3894295. JSTOR3894295.