This page needs a newer browser to run its tools. You can still read everything here.

GIS & Geospatial Data (QGIS, GeoPandas, PostGIS) Module 2

Coordinate reference systems interview questions

Interview questions on coordinate reference systems: short warm-ups, then what each harder question tests, the first step of a good answer and its follow-ups.

15 questions: 5 warm-ups and 10 with model answers · Levels: Fresher to senior

Warm-up questions

Short questions with short answers, each answered in full.

Fresher

What is the difference between an ellipsoid and a datum?

Answered in the lesson Earth's shape: ellipsoids, geoids and datums
Fresher

Why can't a flat map keep both areas and shapes?

Answered in the lesson Map projections and their distortions
Fresher

Why store a CRS as an EPSG code or WKT2 rather than as a PROJ string?

Answered in the lesson EPSG codes, WKT2 and PROJJSON in practice
Fresher

What is the difference between set_crs and to_crs in GeoPandas?

Answered in the lesson Reprojecting data without silent errors
Fresher

When is a planar distance in a projected CRS good enough?

Answered in the lesson Measuring distance and area correctly

Questions with model answers

What each question tests, the first step of a good answer and the follow-ups to expect.

Fresher to mid

A contractor's survey of a pipeline in Assam, delivered as latitude and longitude, sits about 300 m west of your WGS 84 base map, although its shape matches the base map exactly. What is the likely cause, how do you confirm it, and how do you fix the layer?
What it tests

Recognising a datum mismatch from a near-constant offset, confirming it from metadata before changing data, and transforming with a named operation whose accuracy is known.

First step

Measure the offset at several points along the pipeline. A shift of a few hundred metres, nearly the same size and direction everywhere and pointing roughly west, matches numbers on Kalianpur 1975 read as WGS 84: 322 m at Guwahati. Then ask the contractor which datum the coordinates are in before changing anything.
Follow-ups
- The contractor answers "Everest". Is that enough to fix the layer?
- How accurate will the pipeline be after the transformation?
- After transforming, a few points still differ from the base map by about 15 m. Why?
From the lesson Earth's shape: ellipsoids, geoids and datums
Model answer, Pro. Model answers open with any pass; prices are in the notice on this page.
Opening the model answers with your pass…
Model answers open with any MySmartCoPilot pass.
Pro pass · 1 week₹79$2.99
30 days₹179$5.99
1 year₹1,699$45
One payment; a pass never renews. The price shown is the total you pay: no fees or charges are added at checkout.
Passes can’t be bought right now, so the model answers can’t be opened at the moment. If you have a pass, enter your licence key.
Choose a pass I have a licence key
It also opens every other in-depth part in Learn and every Pro tool. Sales are final, except where the law gives you a right to withdraw, for example in the EU and the UK (refund policy).
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Mid to senior

A drone survey near Chennai gives heights from its GNSS receiver, and the site plan's levels above mean sea level are about 92 m higher everywhere. Explain the difference, and say how you would produce levels for the plan that engineers can rely on.
What it tests

Separating ellipsoidal and orthometric heights, using geoid heights correctly, and knowing when a global geoid model is not good enough for engineering levels.

First step

Name the two height systems: the receiver gives h above the WGS 84 ellipsoid, the plan uses H above mean sea level, and h = H + N. With the geoid height N about −92 m near Chennai, a gap of 92 m is exactly what mixing the two produces.
Follow-ups
- Can you subtract 92 m from every height along a 40 km corridor?
- Which geoid model or control would you use for the plan's official levels?
- The receiver can also show a height above mean sea level. Does that solve the problem?
From the lesson Earth's shape: ellipsoids, geoids and datums
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Fresher to mid

An analyst ranks India's districts by forest area, computing areas from a layer in EPSG:3857. A district near Delhi's latitude comes out about 23 % larger than an identical one near Chennai. Explain what happened, how large the error is across India, and what you would compute instead.
What it tests

Recognising Web Mercator's latitude-dependent areal scale, quantifying it, and choosing an equal-area projection or ellipsoidal areas for national statistics.

First step

Check the CRS the areas were computed in. EPSG:3857 is Web Mercator, whose areal scale grows with latitude: 1.060 at Chennai and 1.302 at Delhi in the lesson's program. 1.302 ÷ 1.060 = 1.23, the whole 23 % gap, so the areas, and the ranking built on them, come from the projection.
Follow-ups
- Would EPSG:7755, India's national system, be good enough for the ranking?
- How would you show the analyst the error with their own data?
- The layer has 700 districts. Does correcting each area with one scale factor work?
From the lesson Map projections and their distortions
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Mid to senior

A city keeps all its data in UTM zone 43N. Its new metropolitan region reaches past 78° E, into zone 44N, by about 2° of longitude near 20° N. What happens to lengths and areas at the far edge, and what do you recommend for the region's data?
What it tests

Knowing how UTM's scale grows outside its zone, estimating the error, and choosing between one zone, two zones and another CRS for a region.

First step

Estimate the scale at the far edge. Zone 43N's scale is 0.9996 on its central meridian, 75° E, reaches 1 about 180 km either side and is about 1.0008 at 78° E near 20° N; 2° further east, at 80° E, it is about 1.003, so lengths are 0.3 % long and areas 0.6 % large there.
Follow-ups
- Is 0.3 % acceptable for the region's planning maps? For a utility's pipe lengths?
- What goes wrong if each half of the region is stored in its own zone?
- Which other CRS could the region use, and what would you check before choosing it?
From the lesson Map projections and their distortions
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Fresher to mid

A government shapefile's .prj names a custom "LCC_WGS84" system with no authority code, and a colleague's automatic match, run with a low confidence threshold, suggests a state system. How do you decide which CRS to assign, and how do you show that the choice is safe?
What it tests

Reading a .prj, comparing definitions by the positions they give rather than by names, and letting the provider's documentation decide the code.

First step

Read the .prj's method, datum and parameters, and list registered CRSs with the same method and datum. Then measure what each candidate does to positions: the national EPSG:7755 places points within 5.9 cm of the .prj's grid, while a state system such as EPSG:7756 puts the same points up to 3,772 km away.
Follow-ups
- The metadata sheet names EPSG7755. Is anything left to check?
- Why is the low-confidence match dangerous even though method and datum agree?
- How do you record the decision so the next person does not repeat the work?
From the lesson EPSG codes, WKT2 and PROJJSON in practice
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Mid to senior

Two analysts transform the same layer to WGS 84 with the same EPSG codes, one with a recent desktop GIS and one with an old server library, and their positions differ by metres at some points. How do you find the cause, and how do you make the pipeline's results reproducible?
What it tests

Knowing that tools carry their own copies of the EPSG dataset, diagnosing version and operation differences, and pinning what decides the result.

First step

Compare what each tool actually ran, not the codes: the PROJ version, and so the EPSG dataset version, and the transformation each one chose for the pair of CRSs, with its accuracy. Different EPSG versions can hold different or corrected operations, and a tool without one can fall back to a ballpark operation with no datum shift.
Follow-ups
- Which tool is right?
- How would you pin the result in a pipeline that runs for years?
- The old library cannot be upgraded. What do you do?
From the lesson EPSG codes, WKT2 and PROJJSON in practice
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Fresher to mid

After a colleague "fixed the projection" of a layer of 2,000 clinics in Maharashtra, every clinic appears in the Indian Ocean near the equator, about 2,000 km to the south-west. What most likely happened, how do you confirm it, and how do you repair the layer?
What it tests

Telling a label change from a transformation, diagnosing it from where the points land and from their values, and repairing the layer without new errors.

First step

Look at the stored numbers, not the map. If the "eastings" are around 72 to 80 and the "northings" around 16 to 22, the values are degrees: someone labelled the layer as a UTM zone with set_crs instead of transforming it, so the degrees are read as metres near the zone's false origin.
Follow-ups
- How do you repair it, and in which order?
- What check would have stopped the mistake?
- Some clinics were edited after the "fix". What do you do about them?
From the lesson Reprojecting data without silent errors
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Mid to senior

A pipeline converts old survey points in Kalianpur 1937 for districts of India to WGS 84, and two machines produce positions about 100 m apart for the same points. Why can this happen, and what should the pipeline do instead?
What it tests

Knowing that operations have areas of use, recognising a ballpark fallback or an operation used outside its area, and designing a pipeline that refuses or uses measured control.

First step

Ask each machine which operation it used. For Kalianpur 1937, EPSG has operations to WGS 84 only for Bangladesh (EPSG:1155) and the Moattama area of Myanmar (EPSG:1533), none for India, so one machine may have used one of them outside its area and the other a ballpark operation with no datum shift.
Follow-ups
- Is either machine's result right?
- How would you produce WGS 84 coordinates the project can rely on?
- How should the pipeline behave when no operation applies?
From the lesson Reprojecting data without silent errors
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Fresher to mid

A delivery company computes trip distances in DuckDB with ST_Distance_Spheroid on points built as ST_Point(longitude, latitude). Most trips in Maharashtra come out several per cent short. Explain what happened and how you would fix and test it.
What it tests

Knowing that a function's documented axis order matters, diagnosing a swapped pair from its effect, and adding a test with a distance known from another source.

First step

Read the function's documentation: DuckDB's spheroid functions expect latitude first. With longitude first, every point is read with its axes swapped; for Pune to Mumbai that gives 110,690.41 m instead of 120,138.17 m, 7.86 % short, with no error.
Follow-ups
- Why are the errors different from trip to trip?
- How would you test the fixed query?
- Would switching to ST_Distance_Sphere avoid the problem?
From the lesson Measuring distance and area correctly
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
Mid to senior

Two departments report the area of the same district: one computed it in EPSG:7755, the other on the ellipsoid, and their figures differ by about 3 %. Which is right, how do you explain the difference, and how do you make all district statistics consistent?
What it tests

Understanding the areal scale of a conformal grid, knowing that ellipsoidal areas are the reference, and setting a method for official statistics.

First step

Identify the method behind each figure. EPSG:7755 is conformal, so its areas carry its areal scale, about 0.970 near Pune: the lesson's ward came out 2.99 % small in it and a 10° block 3.15 % small. The ellipsoidal area has no projection error, so it is the reference.
Follow-ups
- Can the EPSG:7755 figures be corrected with one factor?
- How large is the error in Web Mercator, if a third department uses a web map?
- What would you write into the statistics' metadata?
From the lesson Measuring distance and area correctly
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes

The lessons of this module

Earth's shape: ellipsoids, geoids and datums Free lesson, Pro answers: 25 minutes
Map projections and their distortions Free lesson, Pro answers: 20 minutes
EPSG codes, WKT2 and PROJJSON in practice Free lesson, Pro answers: 20 minutes
Reprojecting data without silent errors Free lesson, Pro answers: 20 minutes
Measuring distance and area correctly Free lesson, Pro answers: 20 minutes

All GIS & Geospatial Data (QGIS, GeoPandas, PostGIS) interview questions · The GIS & Geospatial Data (QGIS, GeoPandas, PostGIS) track

Warm-up questions

What is the difference between an ellipsoid and a datum?

Why can't a flat map keep both areas and shapes?

Why store a CRS as an EPSG code or WKT2 rather than as a PROJ string?

What is the difference between set_crs and to_crs in GeoPandas?

When is a planar distance in a projected CRS good enough?

Questions with model answers

A contractor's survey of a pipeline in Assam, delivered as latitude and longitude, sits about 300 m west of your WGS 84 base map, although its shape matches the base map exactly. What is the likely cause, how do you confirm it, and how do you fix the layer?

A drone survey near Chennai gives heights from its GNSS receiver, and the site plan's levels above mean sea level are about 92 m higher everywhere. Explain the difference, and say how you would produce levels for the plan that engineers can rely on.

An analyst ranks India's districts by forest area, computing areas from a layer in EPSG:3857. A district near Delhi's latitude comes out about 23 % larger than an identical one near Chennai. Explain what happened, how large the error is across India, and what you would compute instead.

A city keeps all its data in UTM zone 43N. Its new metropolitan region reaches past 78° E, into zone 44N, by about 2° of longitude near 20° N. What happens to lengths and areas at the far edge, and what do you recommend for the region's data?

A government shapefile's .prj names a custom "LCC_WGS84" system with no authority code, and a colleague's automatic match, run with a low confidence threshold, suggests a state system. How do you decide which CRS to assign, and how do you show that the choice is safe?

Two analysts transform the same layer to WGS 84 with the same EPSG codes, one with a recent desktop GIS and one with an old server library, and their positions differ by metres at some points. How do you find the cause, and how do you make the pipeline's results reproducible?

After a colleague "fixed the projection" of a layer of 2,000 clinics in Maharashtra, every clinic appears in the Indian Ocean near the equator, about 2,000 km to the south-west. What most likely happened, how do you confirm it, and how do you repair the layer?

A pipeline converts old survey points in Kalianpur 1937 for districts of India to WGS 84, and two machines produce positions about 100 m apart for the same points. Why can this happen, and what should the pipeline do instead?

A delivery company computes trip distances in DuckDB with ST_Distance_Spheroid on points built as ST_Point(longitude, latitude). Most trips in Maharashtra come out several per cent short. Explain what happened and how you would fix and test it.

Two departments report the area of the same district: one computed it in EPSG:7755, the other on the ellipsoid, and their figures differ by about 3 %. Which is right, how do you explain the difference, and how do you make all district statistics consistent?