I just noticed two anomalies re the way Therion records survey length.
 
Data recorded like this...
   data cartesian from to northing easting altitude
   7  7a  90  -99  4.5
   7a 9 97 -9 4.5
   9 10 31 -22 0
 
 are reported as zero length surveys in the Therion survey-list output.
A bug?  Depends on point of view I guess, cartesian implies no actual distance measured, so zero survey length, but the definition of two points does have some parallels with a 'shot' and hence survey length.
 
I contrast, 'no survey' leg lengths are recorded as accumulating survey length.
ie
 
flags surface
fix  1 -41.1724     172.4155 572.6
fix  2 -41.20492 172.2789 574.04
fix  3 -41.21414 172.1513 574.04 etc
data nosurvey from to
1 2
2 3
3 4
 
These are reported by Therion as accumulating survey length.
 
I can see that the desirable behaviour of each of these scenarios might be different depending on one's particular intention.
In the case of the particular data where I found this issue, I would prefer the;
- underground cartesian data to accumulate length
- surface nosurvey data to accumulate length, as it in fact does  (I am joining up points from a gps trail)
 
but in general I would expect;
 
-underground nosurvey data to record zero length, because it will generally be a sound or sight connection with no actual survey, hence not surveyed length.
 
I have not checked any of the other data survey formats.
Other peoples thoughts on this?
 
Bruce