Can’t I use CorpsCon to convert survey data from NGVD29 to NAVD88?

In Missouri, yes; but in Louisiana, absolutely NOT! The NAVD88 was developed as a replacement for the NGVD29 because the NGVD29 had become unreliable and out-of-date. The NAVD88 was published for most of the country by 1990, except for those regions of known crustal motion which included Louisiana. In 1992, the NAVD88 was published for Louisiana, but because of the continuing crustal motion of subsidence the lifespan of the NAVD88 benchmark elevations in Louisiana was limited to a few years.

CORPSCON uses the data files developed for VertCon by the National Geodetic Survey. The data files represent a snapshot in time for the instant when the NAVD88 was first published for Louisiana. Those data files are static; they do not change, and they only represent the differences in elevations at certain benchmarks in Louisiana in 1992. Using CorpsCon and/or VertCon to convert survey data in Louisiana will provide conversions that are at least 6 inches in error, an amount that is not acceptable for engineering and design applications. Conversion values determined with CorpsCon and/or VertCon should only be used for pure datum shifts, keeping in mind that subsidence is not accounted for. Additionally, any biases due to regional adjustments or realizations will contribute to the overall errors in the computed shifts. In areas of crustal motion, the computed values are based on the datum relationships at the time to model was created. So when using older NGVD29 epochs such as 1964/65, the differences between NGVD29 and NAVD88 are different than would be for the 1976 epoch.