Geodetic Densification from GNSS Data  

PUBLICATION 02 | TBC CYCLE | GEOCOM Engineering

 

Introduction

The National Geodetic Network of Chile is referenced to SIRGAS-Chile, which is vital for the georeferencing of all geospatial data produced in our country, with epochs that have been periodically updated. To date, the Military Geographical Institute has released four versions of SIRGAS-Chile, associated with the 2002, 2013, 2016, and 2021 epochs. Currently, the IGM is preparing the launch of SIRGAS-Chile 2025, which aims to renew the reference epoch with the objective of calculating more representative coordinates from the perspective of a static datum.

For its part, the densification of the National Geodetic Network responds, among many other reasons, to the need to have a coordinate origin within a particular area to simplify topographic representation operations. Today, this densification is carried out using GNSS, which is the preferred technique for this purpose, offering a variety of possibilities.

In this example, some observation and calculation strategies for densifying SIRGAS-Chile using GNSS observations in a context associated with topographic representation will be reviewed.


GNSS Operation and Observation Context

Every topographic representation, regardless of the technique used, necessarily requires an origin (reference framework) to which the determined coordinates are referred. Usually, this origin is provided by a densification of the National Geodetic Network, established by the IGM, which allows the calculation of coordinates, preferably through a network adjustment.

In this case study, a topographic survey was conducted using GNSS RTK, with the base conveniently set up for this operation. Simultaneously, while the base transmits the necessary data for the mobile GNSS to operate in RTK, a static observation is made to associate this same point with a densification of the National Geodetic Network. In this way, the base receiver, in addition to sending differential corrections, will store a static file in its internal memory, which will be the basis for obtaining its precise coordinates. Specifically, this observation lasted a little over 2 and a half hours.

 


Figure 1. Survey style for the RTK base to simultaneously observe a static point


GNSS Data Processing

TBC is a powerful solution for processing GNSS data to calculate high-precision coordinates. Whether processing GNSS baselines, which are part of a network requiring adjustment, or processing GNSS data absolutely with Trimble RTX, TBC is an excellent tool for GNSS data processing, even recalculating GNSS RTK surveys.

Accordingly, different options for calculating the precise coordinates of the GNSS RTK base station will be evaluated, assessing the differences that arise.

 

Calculation from GNSS Baselines

A GNSS baseline corresponds to a geodetic observation that allows coordinates to be calculated from a known origin. When several baselines are combined, a network is formed, which must be adjusted to calculate coordinates along with their precision determination.

Thus, three closest reference stations from the GEOCOM GNSS Network were chosen to calculate the coordinates of the GNSS RTK base, designated as A1. Accordingly, baselines from CONZ, SNTI, and TALC, which have coordinates referred to SIRGAS-Chile 2025, are processed to obtain the following results, in terms of precision, from the baseline processing report:

.

Figure 2. Excerpt from the baseline processing report

 

This baseline processing allows the calculation of A1's coordinates in different ways. Some of them are:

 Methodology A1 Coordinates
CONZ only -34°59'33.02976"
-71°12'58.21925"
239.173 m
SNTI only -34°59'33.02935"
-71°12'58.21933"
239.217
TALC only -34°59'33.02950"
-71°12'58.21916"
239.183 m
Network adjustment 1 -34°59'33.02950"
-71°12'58.21916"
239.183 m

Table 1. Coordinate determination for A1



Figure 3. Network adjustment using baselines from CONZ, SNTI, and TALC.

 

Another alternative is to use reference stations to densify the geodetic control in the vicinity of the point whose coordinates will be determined. For this, the stations to be used can be planned from the IGM's SIRGAS-Chile website.

 

Figure 4. SIRGAS-Chile website

 

This is how eight reference stations are incorporated into a network that has the same fixed points indicated above as its origin. Thus, new baselines are processed, generating a new geodetic network that requires adjustment:


Figure 5. Excerpt from the baseline processing report

 

 

Figure 6. Network adjustment

 

 


Thus, another set of coordinates for A1 is determined:

 Methodology A1 Coordinates
Network adjustment 2 -34°59'33.02944"
-71°12'58.21915"
239.189 m
Table 2. Coordinate determination for A1

 

Calculation from Trimble RTX

Trimble RTX is a GNSS absolute positioning service that can determine coordinates in both real-time and post-processing. Trimble RTX has an interesting peculiarity: it can determine coordinates instantaneously in terms of epoch. If it is necessary to reduce the instantaneous epoch to a reference, a terrestrial crust velocity model is required, which has already been implemented in TBC for SIRGAS-Chile 2016 and 2021.

In very general terms, the crust of Chile has an average velocity of 2 cm/year. Therefore, in 2025, it is possible to work, with an acceptable approximation for topographic surveys, without a velocity model. In the case of determining the coordinates of A1, the instantaneous epoch associated with the determination of absolute coordinates is 2025.25 (year 2025 and day 93, which is equal to 2025 + 93/365).

Furthermore, TBC can process GNSS data using the Trimble RTX service. If an epoch reduction is required, the necessary reference frame must be configured; otherwise, only WGS84 without any epoch needs to be configured to determine coordinates under the instantaneous epoch.


Figure 7. RTX solution under Trimble Business Center

 

Finally, TBC calculates the following coordinates:


 Methodology A1 Coordinates
Trimble RTX -34°59'33.02938"
-71°12'58.21918"
239.216 m
Table 3. Coordinate determination for A1

 

Conclusions

The coordinates of A1 are calculated using different methodologies, resulting in horizontal differences not exceeding 10 mm and vertical differences not exceeding 40 mm. For cases where GNSS baselines were processed, 2 and a half hours of observation were sufficient to process baselines of approximately 150 km, providing full confidence in the determination of the results.

For its part, Trimble RTX offers greater autonomy by not relying on nearby base stations. However, it is recommended to observe for at least 3 hours to obtain results that are compatible with the accuracies required for a topographic representation project.

 

Figure 8. Horizontal difference for each coordinate determination of A1