What sources do historians use in writing about the last 250 year of India history? How were these preserved?

Sources of the history of the past 250 years:

1. Official records of British administration: Detailed recording, memos, notes, letters etc. were considered important by the British. Every policy, plan, decision etc. was written or printed.

2. Surveys: They were considered essential for effective administration and were of various kinds: revenue survey, botanical survey, forest survey, census etc.

3. Preserving the records: Every administrative office had a record room and certain institutions like archives and museums were made to preserve them.

4. Other records: Apart from official records, there are travelogues, diaries, newspapers, booklets etc. that belong to that period however they are harder to find.