Thanks for the various comments.
DaveReidUK
I've no idea if RadarBox or SBS was used to originate the ADS-B data. Indeed, I have no experience of the RadarBox although, I have to admit, it sounds interesting.
At the end of the day though, whether raw data from the SBS, or corrected data from a RadarBox, would the final altitude have been -200 feet?
Airbubba's post 765 is interesting as far as the correction for QNH is concerned. I wish that SBS would incorporate such a correction. This morning, aircraft taking off from Heathrow were maintaining 5925 feet on my SBS rather than 6000 feet!
snowfalcon2
Sorry if I did not express myself clearly. The point I was trying to make was that the originator of the original graph had already increased the altitude by 200 feet. The total correction (1027-1013) was about 375 feet, hence my comment about raising all altitudes by a further 175 feet. I agree that the aircraft carried on further towards the threshold of 18R. After all, it was still 175 feet or so above the ground and had some forward motion. How far, I do not know and made no claim about knowing this. That is a matter of fact for the investigators. Also, I made no comment about the -4096 fpm vertical speed. This may have been due to an instantaneous sampling rate which was not representative but is not, I feel, relevant.
SBS