geoman-io/leaflet-geoman

Area Preservation when dragging polygons.

Open

#547 opened on Feb 19, 2020

View on GitHub
 (4 comments) (0 reactions) (0 assignees)JavaScript (1,933 stars) (404 forks)batch import
bughelp wanted

Description

When dragging polygons to different latitudes, area is not being preserved. Specifically, when a polygon is created in towards the poles and brought towards the equator, its width is not adjusting appropriately. Height is adjusting though.

Steps to reproduce the issue: Can be done on main site https://geoman.io/leaflet-geoman

  1. Create a circle over a northern country (like the uk)
  2. Create a polygon over the same circle.
  3. Drag the circle to the equator.
  4. Drag the polygon to the same location at the equator.

In theory they should both remain the same size. However they distort in different ways.

Circle and polygon created in north with same shape. image Circle and polygon with different shapes after being dragged to the equator. image

Contributor guide

Tech stack
javascript
Domain
frontend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
leafletmap projectionsJavaScriptgeometry
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50
Research direction
Investigate the drag handling code for polygons in leaflet geoman, likely in `src/edit/L.PM.Edit.Polygon.js` or similar. Compare with circle drag logic which preserves area. Understand how lat/lng coordinates are updated and whether area recalculation uses correct projection. Review comments in the issue for any additional insights.
Area Preservation when dragging polygons. · geoman-io/leaflet-geoman#547 | Good First Issue