Fitting a District on A3: Scale, Source and Symbol Choice
A practical geography class must prepare a map of a district for a cropping-pattern report. The district measures 78 km east–west and 62 km north–south. Available material: four Survey of India toposheets at 1:50,000 that together cover the district; a hand-drawn sketch map at 1 cm = 5 km made by a previous batch; and a recent satellite image print. The final map must be printed on A3 paper, 42.0 cm × 29.7 cm, leaving a 2 cm margin on every side, and the examiner has asked for the largest possible scale that fits. Two students disagree: one wants to enlarge the sketch four times on a photocopier; the other wants to redraw from the toposheets. A third question is which projection to use for a district only 78 km across, and whether block-wise cropping data should be shown by shading whole blocks or by joining points of equal value.
1. Compute the largest scale that actually fits A3 with a 2 cm margin, express it as an RF and as a statement of scale, and identify the mistake a careless student would make.
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Step 1 - Printable area. 42.0 − (2 + 2) = 38.0 cm; 29.7 − (2 + 2) = 25.7 cm.
Step 2 - Test the two dimensions separately.
- East–west: 78 km into 38.0 cm → 1 cm = 2.053 km → RF about 1:205,000.
- North–south: 62 km into 25.7 cm → 1 cm = 2.412 km → RF about 1:241,000.
Step 3 - Take the smaller scale (the larger RF number): 1:241,000, because at 1:205,000 the district would be 30.2 cm tall and would run off the 25.7 cm printable height. The north–south extent, not the longer east–west side, is the limiting dimension.
Step 4 - Round to a standard scale. Adopt 1:250,000, i.e. 1 cm = 2.5 km. Check: 78 km = 31.2 cm and 62 km = 24.8 cm, both inside 38.0 × 25.7 cm, with room for a legend.
The careless mistake is to fit only the longer dimension and choose 1:200,000. That gives 39.0 cm × 31.0 cm - too tall for the sheet. Always check both dimensions against the shorter printable side and pick the smaller scale of the two.
2. Whose method is better - enlarging the sketch four times, or redrawing from the 1:50,000 toposheets? Support your answer with scale reasoning and describe the steps of the better method.
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Compare the sources. The sketch at 1 cm = 5 km has RF 1:500,000. Enlarging four times gives 1:125,000, which is close to the required scale - but enlargement multiplies every defect four times: the draughtsman's misplaced village, the wobbly river, the missing grid. A sketch has no survey control, no projection and no coordinate base, so its errors are not random; they are systematic and become more convincing as they get bigger. Enlargement is therefore unacceptable for a report map.
The toposheets win because 1:50,000 sheets carry surveyed control and a standard grid. Reducing 1:50,000 to 1:250,000 is a five-fold reduction.
Steps.
- Mosaic the four sheets on their grid lines and check edge matching; trim overlap.
- Draw a square grid on the mosaic and a proportional grid (1/5 of each side) on the new sheet - the square method.
- Transfer control points (settlements, road junctions, canal crossings, bridges) first.
- Generalise: keep canals, distributaries, metalled roads, railways, rivers, tank and town boundaries; omit field bunds, minor tracks and individual wells; simplify meanders to their bends.
- Add the block boundaries, legend, scale bar, north arrow and title.
Use the sketch only to check local names, never as geometry.
3. Advise on the projection and on the mapping method: should block-wise cropping data be shown by shading whole blocks or by isolines? Justify both choices.
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Projection. The district spans about 0.7° of longitude and 0.55° of latitude - roughly 78 km by 62 km. Over such a small extent the Earth's curvature produces distortion far below the width of a drawn line, so almost any simple projection will do. Use the Survey of India polyconic convention (or a simple conic with standard parallels placed by the one-sixth rule) so that the map matches the toposheets it was compiled from, and quote the projection in the margin. Mercator must be rejected: it is a navigational projection that exaggerates area away from the equator, and its straight rhumb lines are of no value on a land-use map. Equally, a global projection is needless complexity here; the honest answer is that projection choice matters for a state or country map, not for one district.
Mapping method. Cropping pattern is a statistic attached to an administrative area, the block, and blocks areal units - so a choropleth is correct: shade each block according to the percentage of its net sown area under the chosen crop, with a classed legend and a note on the classification method (equal interval hides variation where one block dominates; quantiles show rank but not magnitude). Isolines are for continuous surfaces - rainfall, temperature, yield per hectare interpolated from points - and would be wrong for block totals because a line drawn between two block headquarters has no meaning. A defensible map may combine both: choropleth for blocks, point symbols for sugar mills and mandis, and a small inset isopleth for rainfall if the report needs it.