Imagine eating a banana slug. Out on the Russian River Near Ukiah I think), they have an annual Bnana slug festival
Ship data is used for the oceans . It is incomplete . There is a voluntary project examining ships old log books to put the data in a bank on computers . Problem is sailing ships only measure the temp in weather . That is, a breeze . They also vary their route considerably .
Steam ships are more regular in their route but still suffer from piecemeal data . If you weren't a big trader than data for your part of the world wont be there . Similarly for land based data, if you are not a rich country then you probably weren't measuring temps every day since 1880 .
We only have the max and min temps for anywhere, and these have been averaged for GW enthusiasts data . The average is not a true reflection of the heat during a 24hr period . You must assume that the temp was equally in halves above and below the average of the max and min . Quite an assumption .
Before it was absolutely essential to have one climate, the earth was considered to have many . We need to measure change, perhaps hourly, at many sites in all the earth's climates especially the oceans . We have only started to do that, in what is a 100,000 yr cycle . How do we eliminate outliers and determine trends if we have a very, very short time of measurement ?
