text: the 1688 comes from a gnis import.  i'd wonder about the references against wgs84, and if there could be 6m error/difference in elevation.
  
  note that i haven't the faintest idea what i'm talking about.  i vaguely recall seeing different local country representations differing from one another and wgs84, but i'd have to take time and refresh my memory.
  
  here the gps receiver barometric altimeters, and the z-axis inaccuracy far greater than the typical 2m single-system waas/egnos-corrected x/y-axis margin of error does not come into play.
  
  a quick google brings me to https://de.wikipedia.org/wiki/Höhe_über_dem_Meeresspiegel where i read the typical difference between systems is at worst around a metre, but later in comparison with wgs84, while the link at https://de.wikipedia.org/wiki/H%C3%B6he_%C3%BCber_dem_Meeresspiegel#Ellipsoidische_H%C3%B6he_(GPS) refers to a difference between wgs84 and the local reference (normal null and so on) between 36 and 50m, north to south for germany.  (any figures like that are sorely missing from the english article on wikipædia, sorry to say)
  
  my attempts to quickly find the usgs topo maps reference datum come up with a blank.  other elevation models refer to (NAD 83 and) NAVD 88, but as the printed topo maps were available well before these years and the scanned maps cover a range of styles i cannot pinpoint to any particular era, my unfamiliarity with north american standards leaves me emptyhanded.
  
  the gnis data was also pre-processed for osm import, but with error(s) in the x/y axis against wgs84.  i'd have to find the original data to see if any z-axis correction was made, should be easy enough.
  
  confused?  so am i.  i think i'll go hiking instead after piling rocks...