ground vs. roof

I took some SQM readings at the CAO with the hand-held meter at the end of my Sunday night observing session. The time was approximately 11:15 PM.

First reading was 19.62. If I remember correctly, I noted as high as 19.80. Last was 19.70.

I examined the data from the roof-top sensor.

2016-02-15T03:30:00.112;2016-02-14T22:30:00.112;0.1;287800;1;19.32
2016-02-15T04:00:00.130;2016-02-14T23:00:00.130;0.1;355153;1;19.55
2016-02-15T04:30:00.108;2016-02-14T23:30:00.108;0.1;428257;1;19.75

The 11:00 PM reading was 19.55 and the 11:30 PM reading 19.75. Very similar!