This can happen because Tacview's default terrain-elevation set (ETOPO30) and CMO's default terrain set (a modified version of SRTM30-PLUS) differ both in granularity/resolution and also in absolute values. So CMO declares that a land unit is at ASL altitude X, whereas Tacview considers that ground elevation at that point is higher, and thus places the ground unit "underground". (The opposite can also happen, showing the unit "floating on air")
This is a pretty common issue in the defence simulation industry, and most such visualizers solve the issue by forcing ground units to appear at the exact AGL level of their own terrain data, even if the telemetry for the unit would place it above or under ground level (this is commonly called "clamping"). Unfortunately Tacview deliberately avoids clamping, as Frantz (its creator) prefers to use the "raw" altitude values.
If this is a critical point for you, feel free to contact him (https://www.tacview.net/product/en/) and ask for clamping support.
In the meantime, you can download the SRTM30 tileset for Tacview (see here: https://www.matrixgames.com/forums/tm.asp?m=4804370), which will provide a better-matching (to CMO) elevation set.
< Message edited by Dimitris -- 5/8/2020 8:02:50 AM >