GPX Reference Point Generator (GPXRPG) User Guide

1. Introduction

GPX Reference Point Generator (GPXRPG) is a web-based program for use by Contributors to the global OpenStreetMap project. GPXRPG was developed to help "armchair mappers" who use OSM's built-in iD Editor to add to the OSM database new features - particularly urban building footprints - by tracing the outline of each building on an aerial imagery background layer. See Aerial imagery quickstart.

As recommended in that OSM Wiki article, before using the building outline tracing method, OSM armchair mappers should ensure that the aerial imagery background they have chosen is closely aligned to OSM's coordinate system - preferably with less than one metre of horizontal error. That is, when latitude-longitude coordinates are expressed in degree-minute-second format, 2 decimal digits would be the minimum required in the seconds field.

The circumference of the earth is approximately 40075 km. So, 1 second of arc = 40075 / 360 / 3600 x 1000 = 30.0 m horizontal distance. Thus, when working with degree-minute-second coordinates, 2 decimal digits in the seconds field will yield 30.0/100 = 0.3 metre precision.

To closely align the aerial imagery background with OSM's coordinate system requires:

a clearly visible target spot in OSM's Aerial Image background layer (perhaps a street light pole, the corner of a building block or a Survey Marker)
an existing reference point feature in OSM's Map Data layer with the precise latitude-longitude coordinates of the aerial image target spot

When these two requirements are satisfied, the armchair mapper can use the iD Editor's Imagery Offset dialog to shift the aerial imagery background layer (if necessary) until the visible target spot in the aerial image layer lies directly below the OSM reference point feature. The user can then proceed with confidence to add new building footprints to the OSM database.

However, when using a Survey Marker as a reference point, you'll find that OSM's built-in iD Editor lacks the native "add point by exact coordinates" dialog that would be needed to create directly the required reference point feature in OSM's Map Data layer. See for an explanation of this omission.

GPXRPG provides a simple work-around for this missing capability by creating and downloading a GPX track file modelling an Arrow with its tip placed exactly at the latitude-longitude position of the chosen target spot. The track file can then easily be added to the iD Editor's map to provide that vital OSM reference point.

Section 2 describes GPXRPG.

Section 3 presents an illustrated walk-through of an armchair mapping session using a Survey Marker and GPXRPG.

2. GPXRPG

GPX Reference Point Generator (GPXRPG) is a web-based program that is accessed at https://www.davidhaycraft.id.au/GPXRPG

When opened, this program displays a data-entry form:

GPXRPG Data Entry Form

Once you have obtained accurate latitude-longitude coordinates for a reference point, you can generate a GPX Arrow file for that point by:

filling in the Display Label form field, to provide an on-map label for the reference point
filling in the Latitude Deg-Min-Sec, Longitude Deg-Min-Sec fields
optionally, filling in the Notes form field, to identify the Authoritative source of the latitude-longitude coordinates
clicking the "Generate Reference Point File" button.

GPXRPG will then validate the form field values, and if no obvious errors are detected, generate and download the GPX file to c:\Users\user\downloads\refpoint.gpx.

GPXRPG-generated GPX file

This GPX file describe a North-facing Arrow about 3.3m high (actually 0.00002 X 0.00003 deg) with its tip placed exactly at the input Latitude-Longitude position.

GPX Arrow

The resulting GPX file is now available to be "dragged-and-dropped" onto the OSM iD Editor map window, making it visible to the user as an accurate reference point for checking the alignment of the aerial imagery background.

OSM iD Editor map with a yellow Survey Marker in aerial image layer closely aligned to GPX Arrow

3. Walk-through of armchair mapping session using a Survey Marker and GPXRPG

3.0 Introduction

This section presents a walk-through of the procedure for adding to the OSM database some building footprints along a suburban road. The procedure is summarised in the following 9-step flowchart.

Armchair mapping session

3.1 Choose Local Region

Choose the region of "Osmand Street, Wanniassa, ACT, Australia." A detailed survey of this region is published at actmapi.act.gov.au.

ACTmapi Home Page

The ACTmapi Survey Map will be used in step 2 (obtain accurate coordinates of selected survey mark) and the ACTmapi Aerial Imagery Map will be used in step 8 (acquire street address info).

3.2 Select Kerbside Survey Control Mark

Conveniently, a kerbside local government Survey Control Mark plaque (SR126) is actually present in Osmand Street.

Kerbside plaque

SR126 is visible in iD Editor's aerial map image default aerial image when zoomed right in to the plaque's location

Plaque visible

A latitude-longitude record for SR126 exists in the ACTmapi Survey Map database

Database record

Its latitude-longitude coordinates are: Latitude = -35° 23' 30.32749", Longitude = 149° 05' 32.80625"

3.3 Use GPXRPG to generate GPX File

Open the http:///www.davidhaycraft.id.au/GPXRPG document and fill in the form like this:

Completed GPXCRPG Form

then click the "Generate Reference Point File" button to create file c:\Users\user\downloads\refpoint.gpx

3.4 Start OSM iD Editor session

Open the OSM website, log in as a Contributor and select Edit mode. The iD Editor window will open, displaying a map with two layers: OSM Map Data Layer (containing Point, Line and Area features); Aerial Imagery Background Layer.

3.5 Add GPX Reference Point to Map

Open File Manager, locate file c:\Users\user\downloads\refpoint.gpx then drag-and-drop it onto the iD Editor map window. The map will now contain a third layer: Custom Map Data displaying the GPXRPG arrow SR126. The map will automatically zoom in to the location specified for SR126.

3.6 Check alignment of Reference Point to Map

Check position of the yellow SR126 plaque in the aerial imagery background layer with respect to the GPXRPG arrow SR126. If more that 1 metre out, adjust the background imagery alignment as follows.

1: click the Background Settings button in the right-edge menu bar to open the Background dialog

Background Settings button and Background dialog

2: use the Background dialog's right-edge scrollbar to scroll down to the Imagery Offset control

Imagery Offset control

3: drag anywhere in the gray area of the control to adjust the aerial image alignment

3.7 Record Imagery Offset parameters

Make a note of the Imagery Offset values chosen in the previous step.

Important! The iD editor only retains these imagery offset values for the duration of the browser session. Each time you start a new session you must repeat these three steps.

3.8 Perform armchair mapping tasks

Click the iD Editor "Area" button and start adding Area features by tracing the outline of the Osmand Street houses visible in the aerial image.

For each new area set Feature Type=House and fill in Address fields.

Adding new House feature

In the ACT, a convenient source for house numbers is ACTmapi's Aerial Imagery map with the Cadastre for Imagery layer enabled.The house numbers appear in pink.

Source for house numbers

When finished all houses in street, check completeness by inspecting range of house numbers visible in iD Editor map with Background=None.

iD Editor map with Background=None

3.9 Save Updates

Click "Save", remembering to include a copy of the Imagery Offset parameters in the Changeset Comment field and click "Upload" to add your new house info to the OSM database.

Changeset Comment field

Final "thank you" message

UG version 1.1 26AUG2026