ENGR10006 Chap.10 QGIS, Coordinate Systems and Elevation Models
QGIS, Coordinate Systems and Elevation Models
The coastal flooding stream works in QGIS, and every result depends on getting coordinates and elevation data right. This chapter explains latitude and longitude, conversion between decimal degrees and degrees, minutes and seconds, and why engineers map the curved Earth onto a flat surface.
It covers UTM and its Australian version MGA, the EPSG codes used in the project, 7855 for Melbourne, 7856 for Rocklea and 4283 for the coastline layer, and planar distance from eastings and northings. It then describes digital elevation models, the difference between terrain and surface models, the 5 metre LiDAR DEM, filling sinks before hydrology, and the BOLTSS elements every assessed map must show.
A worked example converts a coordinate and measures a distance.
What this chapter covers
- 01
Latitude, longitude and DMS
- 02
Projected coordinate systems
- 03
UTM zones and MGA
- 04
EPSG codes in the project
- 05
Planar Euclidean distance
- 06
Raster and TIN elevation models
- 07
Terrain versus surface models
- 08
BOLTSS map elements
Worked example · free
Converting a coordinate and measuring the distance between two flood sites
- +1The minus sign means south. Keep 37 degrees; 0.8258 × 60 = 49.548, so 49 minutes.
- +10.548 × 60 = 32.88 seconds, giving 37°49′32.9″S.
- +1Δx = 320500 - 318790 = 1,710 m and Δy = 5813700 - 5811470 = 2,230 m.
- +1d = √(1710² + 2230²) = √(2,924,100 + 4,972,900) = √7,897,000 ≈ 2,810 m.
Key terms
- Coordinate reference system
- The QGIS term for a coordinate system, abbreviated CRS.
- Easting
- The x coordinate in a UTM or MGA grid, measured in metres.
- Northing
- The y coordinate in a UTM or MGA grid, measured in metres.
- Map Grid of Australia
- The Australian version of UTM, fixed to the Australian plate.
- Digital terrain model
- An elevation model of the bare ground without buildings or vegetation.
- Digital surface model
- An elevation model that includes every object on the ground.
- Watershed
- The area that drains all its water to a common outlet, also called a catchment.
QGIS, Coordinate Systems and Elevation Models FAQ
Why do engineers avoid latitude and longitude for calculations?
Workshop 3 explains that distances and areas on the curved Earth are intensive and error-prone to compute. Projecting onto a flat surface such as the UTM cylinder gives eastings and northings in metres, so ordinary planar formulas, such as the Euclidean distance, can be used over small areas.
Which EPSG code should our Rocklea project use?
Rocklea lies in MGA Zone 56, so the workshop specifies EPSG:7856, GDA 2020 MGA Zone 56. Melbourne work uses EPSG:7855. The workshop flags this as a very important note, because using the wrong zone places every layer in the wrong position.
What does BOLTSS stand for on a map?
Border, orientation, legend, title, scale and source. A north arrow is needed when no recognisable outline shows which way is north, and the scale can be left off a 3D map because its scale is not consistent across the view.
Why must sinks be filled before hydrology analysis?
Sinks are low cells with no outlet that trap simulated water, so flow directions and watershed basins cannot be derived correctly until they are filled. For the flood scripts that follow, however, the workshop asks you to use the original terrain model, because filling removes hollows that genuinely pond water.
Assessment move
Practise the coordinate conversions both ways until they take under a minute, carrying the sign separately and rounding only at the end. Then build a one-page table of the coordinate systems in the project with their EPSG codes, units and where each appears, and check every layer you load against it. Make reprojection a habit: set the project CRS deliberately before adding data, then reproject any layer that differs.
For distance and area, always confirm that the layer's units are metres. Work through the DEM steps in order, filling sinks before hydrology but using the original terrain model for flood scripts. Each time you produce a map, run through BOLTSS before export and keep fonts consistent.
Save screenshots and short notes on each workshop discussion question, because they become evidence for the method and discussion sections of the final report. Finally, write the EPSG code of every layer on your map notes so a teammate can reproduce the work.
