Overcoming Qgis Limitations For Cylindrical Map Projections

What’s Not Working With Cylindrical Projections in QGIS

QGIS relies on the PROJ library for coordinate reference system transformations and map projections. While PROJ supports many different projections, its implementation of cylindrical projections has some key limitations that can lead to inaccuracies and errors in QGIS.

Common Errors and Inaccuracies

When adding layers or changing the project CRS in QGIS, users may notice issues with misaligned layers, incorrect measurements, and distortions in cylindrical projections like Mercator or Equirectangular. This occurs because the proj algorithms used by QGIS do not fully account for some attributes of cylindrical projections:

  • Scale factor – Cylindrical projections distort scale across latitudes, but QGIS does not adjust for this properly
  • Graticule spacing – Spacing between meridians and parallels should vary, but QGIS uses uniform graticules
  • Conformal precision – Conformal cylindrical projections may introduce non-conformal distortions, particularly for larger areas

These limitations can lead to inaccurate layer overlay, length and distance measurements, and unprojected raster visualization when using cylindrical projections in QGIS.

Understanding the Mathematical Limits

The math behind cylindrical map projections relies on some simplifying assumptions that break down at large extents. Projections like Mercator and Equirectangular are defined as cylindrically projecting the globe onto a tangent plane wrapped into a cylinder. This causes the cylinder to intersect the globe’s surface at one tangent point or “standard parallel”.

QGIS forces cylindrical projections to use a single standard parallel, even for large areas that extend far beyond the chosen tangent point. But conformality and scale accuracy degrade rapidly further away from that standard parallel. QGIS does not account for this mathematical limit, leading to growing distortions and inaccuracies for large regions in cylindrical projections.

Working Around Missing Graticules

Many cylindrical projections are defined as having curved latitude and longitude lines that intersect to form graticules. But the proj algorithms used by QGIS plot cylindrical projections using straight, evenly-spaced graticule lines. This causes issues when trying to overlay multiple layers or take measurements:

  • Layers using curved graticules may not align properly
  • Straight lines representing curved sections have inaccurate lengths
  • There are gaps between parallels on polar-aspect projections like North Polar Mercator

To help minimize these issues, users can customize the projection to tweak parameters like standard parallels and use curved grid lines. Graticule intervals can also be adjusted manually to offset the straight grid lines in QGIS.

Customizing the Projection

QGIS relies on the PROJ definition for each CRS to control how map projections are handled. Modifying the PROJ definition used by QGIS provides some workarounds to the limitations of its native cylindrical projections.

Changing the Projection String

The “+proj” parameter defines the projection algorithm used. For cylindrical projections, this base string can be customized to set additional attributes and improve conformality:

  • standard_parallel parameter for defining scale properties
  • lat_1/lat_2 parameters for modifying aspect ratio
  • Overriding ellipse model to use sphere instead

Modifying these parameters manually in the project CRS definition allows for cylindrical projections better tailored for specific use cases in QGIS.

Modifying the PROJ Database

The PROJ database includes entries for thousands of coordinate systems with predefined proj strings. These serve as base definitions that QGIS relies upon. Modifying or adding custom entries into the database provides more control over projections in QGIS:

  • Edit existing projection entries to adjust parameters like standard parallels
  • Define completely custom projections with specialized proj strings
  • Use proj definitions from other geospatial tools as starting points

By tailoring the PROJ database, users can work around issues with QGIS’s implementation of cylindrical projections.

Adding Custom Formulas

The “+proj=pipeline” parameter allows stringing together multiple proj operations into one CRS definition. This can be used to add custom transformations to better approximate curved graticules:

  • Apply sine/cosine functions to latitudes/longitudes to bend grid lines
  • Add intermediate latitudinal transformations
  • Insert azimuthal equidistant steps for polar projections

Creating a proj pipeline with custom transformations provides a way to achieve more accurate cylindrical projection rendering in QGIS.

Accurate Measurements

The limitations of cylindrical projections in QGIS make taking accurate measurements challenging. But with some careful parameter configuration, data manipulation, and processing, improved measurement precision is possible.

Adjusting for Scale Distortion

Changing the CRS geographic transformation can reduce scale distortions and allow for more accurate length and distance measurements:

  • Use ‘+geoc’ parameter to force spherical calculations
  • Set appropriate standard parallels for each usage case
  • Define multiple zones with different standard parallels

These changes better account for cylindrical map scale properties in QGIS to improve measurement precision.

Georeferencing Raster Layers

Georeferencing applies real-world coordinates to image data, allowing proper spatial alignment in QGIS. For cylindrical projections, control points can transform rasters to fit each area of interest:

  • Add control points at known locations across study extent
  • Use 2nd order polynomial transformation for global data
  • Apply Helmert transform to shift polar-aspect cylindrical imagery

Georeferencing raster data can compensate for projection distortions when overlaying and taking measurements.

Setting Appropriate Parameters

Measurement tools like the measure line and field calculator rely on the project’s coordinate reference system. Configuring parameters appropriately improves their accuracy:

  • Set preferred measurement ellipsoid through “+ellps” parameter
  • Specify customized false easting and northing values
  • Enable “respect layer CRS” option in field calculator settings

Tuning these settings helps QGIS better translate geographic data into projected Cartesian coordinates for measurements.

Better Visualizations

The cylindrical projection limitations in QGIS often result in suboptimal map outputs. However, users can employ various visualization techniques to produce higher quality and more accurate maps.

Tweaking the Map Canvas

The QGIS map canvas offers display settings that can improve cylindrical projection quality:

  • Increase output DPI for crisper graticule lines
  • Override grid style to use curved coordinate lines
  • Set custom intervals for parallels and meridians

Optimizing the map canvas settings helps cylindrical projections better visualize without degradation or artifacts.

Applying Data-Defined Overrides

Data-defined overrides allow substitute coordinate values from layers to transform visualization:

  • Draw points using latitude/longitude fields for coordinate lines
  • Label features using custom coordinate attributes
  • Style layers using implicit Cartesian data fields

By overriding default coordinates, cylindrical projections can achieve better conformality and shape representation in QGIS visualizations.

Exporting Other Formats

Exporting maps allows applying additional post-processing transformations:

  • Rasterize layers to compress graticule complexity
  • Slice output into separate regional pieces at different standard parallels
  • Reproject to an alternate coordinate system better suited for visualization

Alternate export formats provide more flexibility to improve cylindrical projection quality compared to the native QGIS map canvas.

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