Coordinate system transformation : ITRF, ETRF, WGS84

Clear introduction about Global and local referential (Lantmateriet) : “Positions determined by the GNSS method Precise Point Positioning (PPP) are in the same reference frame as the orbits, i.e. usually a realization of ITRS, e.g. ITRFyy, IGSyy or WGS84, where “yy” represents the year of the realization. The coordinates change with time in the ITRS realizations, because of the plate tectonics. Hence,the determined coordinates are given in the epoch of the observations. For practical applications like mapping and referencing spatial data , a static system/frame, which does not change with time, is desired. For this purpose, ETRS89 has been developed for Europe. ETRS89 coincides with ITRS at epoch 1989.0.”

Another simple reading about dealing with ITRS, ETRS and WGS84 is at Confluence website.

Nota : the transformations made by GIS software from WGS84 to local referential are precise at 1 meter only. For centimeter accuracy, use a geodetic software taking into account the velocities of the ITRF referential relative to the local referential (e.g. ETRF for Europe).

There are many WGS84 realizations. The latest compares with ITRF08 and ITRF14, see table below:

YearRealization (Epoch)For all practical purposes equivalent to:
1987WGS 1984 (ORIG)NAD83 (1986)
1994WGS84 (G730)ITRF91/92
1997WGS84 (G873)ITRF94/96
2002WGS84 (G1150)ITRF00
2012WGS (G1674)ITRF08
2013WGS (G1762)Compares to ITFR08 within 1cm Root Mean Square (RMS) overall
Table of WGS84 compared to ITRF from https://www.e-education.psu.edu/geog862/node/1804

For US, WGS84 (G1762) is equivalent at 1cm to ITRF14. For France also ITRF14 ~ ITRF08 at less than 1cm, the transformation is given here http://itrf.ensg.ign.fr/trans_para.php

For France, the official referential is RGF93, which in its latest version is defined as ETRF2000 (epoch 2009.0) ( https://geodesie.ign.fr/index.php?page=rgf93 ).

Position and velocities of the IGS stations in ITRF2014 at epoch 2010 is given at this adress http://itrf.ensg.eu/ITRF_solutions/2014/doc/ITRF2014_GNSS.SSC.txt

Tools to Convert between referentials

Reference online tool to convert between ITRF and ETRF : http://www.epncb.oma.be/_productsservices/coord_trans/index.php

Coordinate transformation software for France : Circé IGN

Coordinate transformation software : PROJ

Example : converting from ITRFxx (epoch XXXX) to RGF93

We have made a GNSS survey in May 2020 that we want to convert to France official referential in a projection. The GNSS referential will then be ITRF14 (epoch 2020.1), and the French referential will be RGF93 with the associated projection Lambert93. In order to do so, we need to make some referential transformation, and also some conversion between cartesian coordinates, geographic coordinates and projected coordinates.

To transform ITRF14 (epoch 2020.1) to RGF93, use this site and choose ETRF2000 (epoch 2009) as equivalent to RGF93 (see this post), for the velocities you must choose the one of the nearest IGS station (positions and velocities given at ITRF official site or Euref site). Then to transform cartesian to geographic coordinates, use Circé IGN software. And finally, use also Circé to convert to projected coordinate system.

More information

GNSS processing with open-source softwares

The main methods of GNSS processing are (1) the classical differential correction with a permanent base station, and (2) the Precise Point Positionning (PPP) method that does not require a base station and uses precises clock and ephemeris data.

The tools used can be softwares or web-services. Among software we can distinguish between commercial softwares (e.g. Trimble Business Center TBC), open-source softwares (e.g. RTKLib), and scientific softwares (Bernese, Gamit/Globk). We will introduce RTKLib and web-services.

RTKLib

RTKLib is the main open-source library and software to process GNSS data. It can do conversion, post-processing, navigation, plotting, and so on.

Note : Several versions of the software exist and they behave somewhat differently. I’ll try to describe them one by one but you’d better check carefully your results compared to another “official” reference in order to validate your process.

Versions of RTKLib :

  • Official software, with actual version 2.4.2. Does not work for me in post-processing as it is not possible to load base station Rinex data.
  • Emlid Fork here. Should be best for conversion of UBX (Emlid raw format) to RinexFor. For what I can say, it worked for conversions, for post-processing of the base station, but it was not stable for post-processing of rover.
  • RTKLib_Explorer fork here. Located on the really rich blog about GNSS and RTKLib named RTKLibExplorer, this tool works well and is well documented. The setting of parameters for static processing and obtain a single position is hard (set ON for “Output Single for Sol outage” and set a value for Max Sol Std)

Complementary tools for dealing with POS files from RTKLib and CSV files from ReachView :

PPP web-services

Static GNSS correction with web-service

This processing is done with static GNSS correction using a network of permanent stations.

RGP IGN http://rgp.ign.fr/SERVICES/calcul_online.php , good for France, gives results in many coordinate systems including RGF93-Lambert93

GNSS complentary data and tools

GPS and Glonass ephemeris data, along with atmospheric data at https://kb.igs.org/hc/en-us/articles/115003935351

and also here (search more simple) https://webigs.ign.fr/gdc/fr/product/format#ephem

and also here (search simple too) https://cddis.nasa.gov/Data_and_Derived_Products/GNSS/orbit_products.html

IGS Antenna calibration file (does not contain Emlid Reach RS2) : igs14.atx

List of IGS stations : here

GPS Calendar to get the GPS week : here or at IGN site

Julian calendar to get GPS day : here or at IGN site

Offset between GPST (GPS Time) and UTC : here

More information

Filtering of DEMs

Several algorithms can be used to filter DEMs. Most simple and common are:

A less common but very recognized and efficient:

Examples of such filters on an noisy DEM :

original_dem
original_dem

Click on caption to enlarge. r : radius