For international harmonization, US approach charts have been migrating to removing the TDZE altitude from approach plates and moving towards THRE as new approaches were designed and older ones were periodically updated. To date, over 4000 approaches have undergone this change, this is about 20% of the total.
The TDZE (TouchDown Zone Elevation) is defined as the highest elevation in the first 3000 feet of the runway starting at the threshold. The THRE is defined as the Threshold Elevation. Anytime the THRE is the highest point in the TDZE, they are the same. However, when the highest point is further down the runway, TDZE can be higher than THRE. If the runway meets standards, this can be as much as a 20 foot difference. In some rare cases where the runway is provided a variance, it can be more.
The THRE is not considered an operational altitude as one does not land on the threshold, but inside the TDZE. In US regulations, THRE is not mentioned, but TDZE is. In particular, on an approach (not Cat II or III) where the approach light system being in sight is used as the sole means of permitting descent below the DA/MDA, US regulations specify that they may only be used for the continued descent to an altitude 100 feet above the TDZE without meeting other criteria. With only THRE shown on the approach chart, this does not provide the data necessary to comply with the regulation.
The net result is that AeroNav has stopped adding THRE to charts and will be putting TDZE back on those that have THRE.
