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Delphi FMX Leaflet Plotter using OSM Maps

Posted on 31st Jul 2018 in Delphi FMX


Project Background

 This is a FMX project to View and Plot latitude and longitude points on the OSM (Open Source Mapping layers) in the browser component. Use the excellent ‘Scriptgate’ library created by Jun HOSOKAWA (install through the Gettit Manager) to make call-backs to the leaflet plotting library using pure Delphi to extract the synchronized plotted points and toggle editing /read- only on the map. Note! No Google Maps are used and only free Open-Source libraries are used to create this example.

 By making use of the open source JavaScript libraries, all the processing is performed in the browser without having to create an extra Delphi layer to code and perform these functions. This makes it light enough for use in a Mobile Application.


Components used

 Delphi FMX standard components:- Toolbar, Browser, buttons and memo. This could easily be used in a VCL application.

  • js is a mature open-source mapping JavaScript library which has a huge following with all sorts of open-source supporting libraries that will provide a solution for any type of mapping requirement.
  • Leaflet plotter is found in Github is just one of these add-on libraries which allows plotting onto a leaflet map.
  • Scriptgate is a fantastic call-back light-weight library that allows Delphi to communicate with the Browser by overcoming the CORS restriction in the cross platform browser component. First install the component through the Gettit Manager and only then can you compile the example.

 Difficulty Low

An understanding of HTML, CSS and JavaScript would help you to understand the logic of the application design.

 Project structure

Delphi Project

 The project consists of a main form and a HTML page plotter.html. The html page acts as the Interface page for both Delphi and JavaScript to communicate with and also will be the page that will be loaded into the browser to show the OSM map.


 This holds all the js and css files.

  • js and leaflet.css are part of the leaflet JavaScript mapping library.
  • Leaflet, plotter.js and plotter.css are from the open-source plotting library which has all the plotting functions and the look and feel of the plotted line and plotted point.


This holds standard images for leaflet.js. In this application these images are not used and the images can be removed.



The code of the Original Leaflet Plotter library code for the plotter.js and plotter.css has been adjusted to show a plotted blue- line with a round blue geo-point icon. The original featured a bland square gray- icon.


As all the work is performed in the html file.  Let’s take a good look at the two main functions that are required to interface with the Delphi code.

The first function toggles the plotting process from Read-Only to Edit and Visa-Versa and calls an existing function  ‘plottedPolyline.setReadOnly()’ within Plotter.js. This function is set up to allow Delphi to call this function directly using ‘Scriptgate’.


    // Trigger for toggling readonly property

      var readOnly = false;
      function toggle(actual){
      if (actual== "read"){
      } else  if (actual== "edit"){
       if (actual== "read"){
       return 'Editing';
       }else if (actual == "edit"){
        return 'Read Only';  }


Back in the FMX main form the ‘editbuttononclick ‘ method becomes a toggle button which will toggle  to read / or edit and this will call the above function using the ‘FScriptGate.CallScript’ function.


procedure TForm4.btneditClick(Sender: TObject);
if btnedit.text = 'Read Only' then
procedure(const iResult:string)
  btnedit.Text:= iResult;
if btnedit.text = 'Editing' then
procedure(const iResult:string)
   btnedit.Text:= iResult;


The second function will call all the latitude and longitude points sequentially form plotter.js. The plotter.js library uses leaflet.js to track all ‘latlng’ changes and updates. The function within Plotter.js called is ‘plottedPolyline.getLatLngs()’. All the points are extracted into a variable using a ’ for ‘ statement.


      function getpoints(){
      var Points = plottedPolyline.getLatLngs();
      for(var i=0; i < Points.length; i++) {
      if(map.getBounds().contains(Points[i])) {
           return Points;


In the FMX main form the buttononclick method calls the above function using ‘FScriptGate.CallScript’ function and fills the memo with the latitude and longitude points. The points are loaded as text(string).


procedure TForm4.Button1Click(Sender: TObject);
 procedure(const iResult:string)
  memo1.Text:= iResult;


Running the Project

Ensure that you have connection to the Internet before running the Application as it needs to call the OSM Maps to download the correct tile(s) to be viewed in the browser.

The plotting is essential a new map overlay.  A left click will add a plotted point and update the plotted points list. This point can be repositioned with left mouse-down and dragged.

Double clicking the point will delete the point and update the internal plotted points list.

 Summary – Where to Next?

Whilst this application is barebones in design. With a little extra coding on your part, it can become a fully-fledged application by adding functionality such as extracting to GPX coordinates and adding elevation data. Either Delphi or JavaScript would work, however I would suggest making use of the light- weight JavaScript libraries for designing up an Android / IOS app. For the purest Delphi coder, the uploaded data can be coded using Delphi GPX libraries and ideal for a VCL or FMX Desktop Windows App.

I am sure you found this as interesting as I enjoyed putting this code together.


Feel free to download the code from Here

Download Code


Happy Coding


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