The helicopter survey is considered by some to be the most accurate census for determining populations on a given unit of land. The total number of deer recorded on helicopter surveys should not be considered a complete count of all deer. Studies by Texas A&M University, Kingsville indicate that accuracy of helicopter surveys in South Texas is fairly consistent, but they can underestimate deer density by 60-70%. The information gathered from this type of survey should be used as population trend information and for the preparation of annual harvest recommendations with the understanding that the deer density figures are probably conservative.
You can get more information on deer counts on the WC4EB.org page – https://www.wc4eb.org/what/the-deer-count/
Note: the type of land cover in south Texas and they still say the count can be off by as much as 70%;