What is a 3D view?
A 3D view draws layers over a terrain surface that is taken from an elevation model. Imagery and vector layers are draped over the terrain, and some vector layers can be extruded, such as building footprints with a height field. You then move a camera around the scene.
A 3D view helps when the shape of the land is part of the story, as with a valley that channels a road or a ridge that blocks a view. It shows the same data as the 2D map. It does not create new measurements.
Terrain
The terrain comes from a digital elevation model. The type matters. A digital surface model (DSM) includes buildings and tree canopy. A digital terrain model (DTM) shows bare ground. Over a dense city, a DSM gives rough ground and bumps where buildings stand. Check which one you have, and say so in the caption.
The elevation raster must also be in sensible units. Heights in metres over a horizontal extent in degrees give a distorted scene unless the software converts them.
Vertical exaggeration
Relief in a city may be small against the width of the area. Bengaluru, for example, varies by about 210 metres across 30 kilometres, which looks nearly flat at true scale. Multiplying the heights by a factor makes valleys visible. This is a display choice and distorts slopes, so the factor should appear in the caption. Use a small factor if you want an honest impression of steepness.
Draping imagery
A satellite image draped on terrain looks realistic, but it is only as accurate as its own positioning. If the image was not orthorectified, buildings and tall trees can lean away from where they stand in the model. The model and the image can each be correct and still disagree.
Animation
Camera paths
A camera animation is built from keyframes. Each keyframe stores a camera position, direction and a time. The software moves smoothly between them. Exporting writes a numbered set of image frames, and a video tool such as ffmpeg joins them into a video.
A fly-through is a presentation tool. Keep it slow with few turns, and finish on the feature you want people to remember.
Time
A temporal animation steps through time instead of through space. Layers with time information, such as monthly vegetation index images, play in sequence in the 2D map.
Charts
A chart answers one question. Pick the chart type from the question.
| Question | Chart |
|---|---|
| How do categories compare? | Bar chart |
| How is a value spread? | Histogram or box plot |
| How are two values related? | Scatter plot |
| How does a value change over time? | Line chart |
Label the axes with units, and show the number of observations when a ranking depends on how much data stands behind each value. A park with a high index based on a few cells is less reliable than one with thousands.
Reports
A report page ties a map, a chart and a short caption together. The caption should give the data source, the date, the method and any limits. For a series of pages, such as one per district, an atlas repeats the layout and changes the title and caption for each.
Example: greenest parks
The QGIS tutorial draws a 3D view of Bengaluru from a 30 m elevation model with Sentinel-2 imagery draped over it, with a vertical scale of 8 to make the valleys visible. It then charts the mean NDVI of the twelve greenest named parks, leaving out parks with fewer than 100 pixels, and places the map, the chart and a caption on a single layout.
Common mistakes
- Exaggerating the vertical scale without saying so.
- Treating a draped image as proof of 3D accuracy.
- Using a DSM and describing it as ground level.
- Exporting animation frames at a size so large that the export takes hours.
- Ranking features on a statistic that rests on very few cells.
- Publishing a chart without its source, date and units.
Glossary
| Term | Meaning |
|---|---|
| DSM | Elevation model including buildings and vegetation |
| DTM | Elevation model of the bare ground |
| Vertical exaggeration | A factor that multiplies heights in a 3D view |
| Drape | Laying an image or layer over a surface |
| Keyframe | A stored camera position at a point in time |
| Extrusion | Raising a 2D feature to a height in 3D |
Practise it
The tutorial 3D maps, animation, graphs and reports builds the 3D view, the chart and the report page.
Test yourself
1. Why should you state the vertical exaggeration?
It stretches heights and makes slopes look steeper than they are, so readers need the factor to judge the relief.
2. What is the difference between a DSM and a DTM?
A DSM includes buildings and trees. A DTM shows the bare ground.
3. How do you make a video from a QGIS camera animation?
Export the frames as images, then join them with a video tool such as ffmpeg.