We report signal-to-noise ratio { \mathrm { SNR } } \simeq 100 optical spectra for four DA white dwarf stars acquired with the GMOS spectrograph of the 8m Gemini north telescope .
These stars have 18 < g < 19 and are around T _ { \mathrm { eff } } \sim 12000 K , were the hydrogen lines are close to maximum .
Our purpose is to test if the effective temperatures and surface gravities derived from the relatively low signal-to-noise ratio ( \langle { \mathrm { SNR } } \rangle \approx 21 ) optical spectra acquired by the Sloan Digital Sky Survey through model atmosphere fitting are trustworthy .
Our spectra range from 3800Å to 6000Å , therefore including H \beta to H9 .
The H8 line was only marginally present in the SDSS spectra , but is crucial to determine the gravity .
When we compare the values published by Kleinman et al .
( 9 ) and Eisenstein al .
( 6 ) with our line-profile ( LPT ) fits , the average differences are : \Delta T _ { \mathrm { eff } } \simeq 320 K , systematically lower in SDSS , and \Delta \log g \simeq 0.24 dex , systematically larger in SDSS .
The correlation between gravity and effective temperature can only be broken at wavelengths bluer than 3800Å .
The uncertainties in T _ { \mathrm { eff } } are 60 % larger , and in \log g larger by a factor of 4 , than the Kleinman et al .
( 9 ) and Eisenstein al .
( 6 ) internal uncertainties .