What a mauza naksha is

A mauza is a revenue village — the unit the land record is kept by — and its naksha is the cadastral map drawn when the village was surveyed. Every plot (khesra, खेसरा) has a number on the map, and the same number in the khatiyan gives its owner, its class of land and its recorded area, the rakba (रकबा). The map and the register together are the record; the map shows where and what shape, the register shows whose and how much.

Bihar has up to three maps of the same village:

CS — Cadastral Survey (कैडस्ट्रल सर्वे), made between about 1900 and 1920 under the Bengal Tenancy Act. Still the base record in much of the state, and the most common sheet on the portal.
RS — Revisional Survey (रिवीजनल सर्वे), made district by district from the 1950s onwards; where it was completed and published, the RS khatiyan replaced the CS one and the plots were renumbered.
Chakbandi (चकबंदी, consolidation), where holdings were consolidated; these maps show the new chaks with their own numbers.

A newer map does not always cover a mauza, and an older one is often still what a deed refers to. Naapi's library labels each sheet with its survey, and the portals page explains where each kind comes from.

The scale

Bihar village maps were drawn at 16 inches to the mile, that is 1 : 3,960 — one inch on the sheet is 330 feet on the ground, one millimetre is about 4 metres. Dense village sites were sometimes drawn at 32 inches to the mile (1 : 1,980). The scale is printed in the legend, usually with a bar in chains: the survey chain (zarib) is 66 feet, 100 links (kadi) of 7.92 inches.

Two things follow. First, a printed line is about 0.3 mm thick, which at 1 : 3,960 is more than a metre of ground; a boundary on the map is a band, not a hair, and any measurement from it carries that width. Second, the sheet you have is a scan or a print of a copy of a hand-drawn original, and paper stretches and shrinks: a sheet may be a percent longer one way than the other. Naapi lets you set the scale from a plot whose rakba you trust (Known area) precisely so that these effects can be checked and absorbed.

Sheets and the sheet index

A small mauza fits on one sheet; the portal names that file E (entire). A larger one is cut into numbered sheets — 1, 2, 3 … — and the first sheet, or a separate index, shows how they fit together. In Naapi the Join tool puts sheet 2 beside sheet 1 by matching two points that appear on both; the Overlay tool lays a CS sheet over an RS sheet of the same ground.

Reading the sheet

North is at the top unless an arrow says otherwise. The Rotate control in Naapi turns a sheet that was scanned askew.
The mauza boundary is the heavy line around the whole map, with the names of the neighbouring mauzas written outside it, often with their thana numbers.
Plot boundaries are the fine lines; every enclosed field carries its khesra number. A number with a dash or a slash (1052/2) is a plot split after the survey.
Roads, paths, canals, ahar and pyne (आहर, पइन), tanks, wells, temples, the village site (basti) and the burial ground have their own symbols; the legend on the sheet lists them.
Thana number (थाना नंबर) in the title is the number of the mauza within its revenue thana — the "Thana No." in every portal file name. It identifies the mauza; the khesra identifies the plot inside it.

Why a measured area differs from the rakba

Measure a plot carefully on its map and the figure will rarely equal the rakba in the register to the dhur. The usual reasons, roughly in order of size:

The scale. Set 1 : 4,000 where the sheet is 1 : 3,960 and every area is 2% low; a photograph or a screenshot has no scale at all until it is calibrated. Check with a plot of known area.
Which katha. The record is in acre and decimal; the bigha–katha–dhur figure someone quotes depends on the local katha (see Units). 1 bigha 4 katha is 104.8 decimal in Vaishali and 75 decimal in Patna.
The plot has changed. Land is sold, divided among heirs and mutated in the jamabandi for a century after the survey, while the map keeps the original outline. The rakba you hold may be a share of the plot on the map.
Line width, paper and scan. A boundary drawn a metre wide, a sheet that has stretched, a scan that is not square — each moves a boundary a fraction of a percent of the plot's width, more for small plots than large.
Rounding. The survey rounded to the dhur or to two places of a decimal; a small plot's rakba is rounded more heavily in proportion.
The trace. On a scan the outline is traced through pixels; on a portal PDF Naapi takes the drawn lines themselves, which is why those sheets measure closer.

Naapi shows the shoelace area with two independent checks (triangles and the kanghi ordinates) so that the arithmetic can be trusted; the differences above are all in the outline, the scale and the record, not in the arithmetic.

Checking a sheet in five minutes

Open the sheet and read the printed scale; set it under 3 Map scale.
Find a large, regular plot whose rakba you know and measure it. Within a percent or two: the scale is right.
If not, use Known area on that plot and measure the plot you care about at the corrected scale.
Compare the plot's outline with the ground: are there sub-plots, a road cut through, a shifted bund?
Note the figure with its survey (CS, RS) and its scale on the report, so the next reader knows what was measured.