The first Sultan to adopt the principle of measurement of cultivable land for determining the land revenue was Alauddin Khalji.

Explanation:
During the Delhi Sultanate period, land revenue played a crucial role in the administration and governance of the empire. Alauddin Khalji, who ruled from 1296 to 1316 AD, introduced several reforms to strengthen the empire and enhance its revenue collection. One of his significant reforms was the measurement of cultivable land for determining land revenue.

Importance of Land Revenue:
Land revenue was the primary source of income for the Sultanate. It was collected from the cultivators, who were required to pay a specific share of their agricultural produce as revenue. The measurement of land helped in determining the accurate amount of revenue that could be collected from each plot of land.

Alauddin Khalji's Reforms:
Alauddin Khalji introduced the system of measurement of cultivable land known as the 'Ghalla Bakshi' system. Under this system, officers known as 'Ghalla Bakshis' were appointed to measure the cultivated land and calculate the revenue accordingly. The measurement was done using units called 'Bigha' and 'Gaz'.

Advantages of Land Measurement:
1. Accurate Revenue Calculation: The measurement of land allowed the authorities to determine the exact area under cultivation, helping in the accurate calculation of revenue that could be levied.
2. Uniformity: The introduction of a standardized measurement system ensured uniformity in revenue collection across different regions of the empire.
3. Prevention of Revenue Evasion: Land measurement helped in preventing revenue evasion by ensuring that cultivators could not underreport the extent of their cultivated land.
4. Effective Administration: The measurement system facilitated better administration and governance by providing reliable data on land holdings, agricultural productivity, and revenue collection.

Impact of Alauddin Khalji's Reforms:
Alauddin Khalji's reforms, including the measurement of cultivable land, resulted in a significant increase in revenue collection for the Sultanate. This enhanced revenue base provided the Sultanate with the necessary resources to strengthen its military, carry out public welfare measures, and maintain a stable administration.

In conclusion, Alauddin Khalji was the first Sultan to adopt the principle of measurement of cultivable land for determining land revenue. His reforms played a crucial role in enhancing revenue collection and strengthening the empire during the Delhi Sultanate period.