Changing GIS Coordinates / Geodetic Datums

[RCA]

FYI. (Something good for California appraisers to know ...)

ME: Can the GIS coordinates of a location on earth change?

ChatGPT: "Yes, the GIS coordinates of a location on Earth can change due to several natural and human-made reasons. Here are the primary factors contributing to such changes:

1. Plate Tectonics: The Earth's lithospheric plates are constantly moving, albeit very slowly. This movement can cause the coordinates of locations on these plates to change over time. For example, the Pacific Plate, on which part of California sits, moves northwestward relative to the North American Plate.
   
   
2. Land Subsidence and Uplift: Certain processes such as groundwater extraction, mining, and natural compaction of sediment can cause land to subside, while geological processes like isostatic rebound can cause land to rise. These changes can alter the elevation and horizontal position of a location.
   
   
3. Glacial Rebound: In regions previously covered by glaciers, the land is still rebounding from the weight of the ice that has melted since the last Ice Age. This process of isostatic adjustment can change both the elevation and latitude of locations.
   
   
4. Earthquakes: Significant seismic activity can abruptly change the landscape, shifting the coordinates of affected areas. For example, the 2011 earthquake in Japan shifted the entire country's main island of Honshu by several meters to the east.
   
   
5. Human Activities: Large-scale construction projects like dams or reclamation can physically alter the landscape, influencing the coordinates of locations. Additionally, changes in how coordinates are measured or defined by geodetic systems (such as updates to GPS technology or shifts from one datum to another) can also alter the recorded coordinates of a location.

These factors make it essential for geospatial data to be periodically updated to maintain accuracy, especially in dynamic regions susceptible to environmental changes or human intervention."

ME: How do surveyors handle such changes?

ChatGPT: "Surveyors handle the changes in GIS coordinates due to natural phenomena and human activities by employing a variety of methods and technologies to ensure that their measurements remain accurate and up to date. Here are some of the primary approaches:

1. Continual Monitoring and Re-Surveying: In areas known to be susceptible to significant changes (e.g., earthquake zones, subsiding cities), surveyors frequently re-survey to update the data. This ongoing monitoring helps in managing and anticipating significant shifts in the landscape.
2. GPS and Satellite Technology: Modern surveying heavily relies on Global Positioning System (GPS) technology, which can provide precise and continual updates on location data. GPS, along with other satellite-based systems like GNSS (Global Navigation Satellite System), allows surveyors to detect and record even minor changes in position.
3. Geodetic Datums: Surveyors use geodetic datums, which are reference systems that provide a frame of reference for measuring locations on Earth. As our understanding of the Earth’s shape and size improves, or as shifts in the Earth’s surface occur, these datums are periodically updated. Surveyors must adjust their measurements to align with the current datum to ensure accuracy.
4. InSAR (Interferometric Synthetic Aperture Radar): This technology uses radar images from satellites to detect changes in the Earth’s surface. It’s particularly useful for monitoring slow, gradual movements, such as those caused by land subsidence or volcanic activity.
5. LIDAR (Light Detection and Ranging): LIDAR uses light in the form of a pulsed laser to measure variable distances to the Earth. This technology provides high-resolution maps of the Earth's surface, which are essential for detecting changes over time.
6. Robotic Total Stations: These are advanced surveying instruments that automatically track a moving prism and can operate continuously. They are often used in monitoring applications to detect and record real-time changes in the position of structures or landforms.
7. Collaboration with Geologists and Engineers: Surveyors often collaborate with other professionals to understand the underlying causes of geographical changes, such as plate tectonics or groundwater extraction, which can inform more effective surveying strategies and adjustments.

By integrating these methods and technologies, surveyors can adapt to changes in the Earth's surface, ensuring that their data remains reliable for construction, land development, navigation systems, and other applications that require precise geographical information."

https://oceanservice.noaa.gov/education/tutorial_geodesy/

At its most basic level of definition, the horizontal datum is a collection of specific points on the Earth that have been identified according to their precise northerly or southerly location (latitude) and easterly or westerly location (longitude).

At its most basic level of definition, the horizontal datum is a collection of specific points on the Earth that have been identified according to their precise northerly or southerly location (latitude) and easterly or westerly location (longitude).

oceanservice.noaa.gov

At its most basic level of definition, the horizontal datum is a collection of specific points on the Earth that have been identified according to their precise northerly or southerly location (latitude) and easterly or westerly location (longitude).

At its most basic level of definition, the horizontal datum is a collection of specific points on the Earth that have been identified according to their precise northerly or southerly location (latitude) and easterly or westerly location (longitude).

oceanservice.noaa.gov

The Vertical Datum: Global Positioning Tutorial

The vertical datum is a collection of specific points on the Earth with known heights either above or below mean sea level. Near coastal areas, mean sea level is determined with a tide gauge. In areas far away from the shore, mean sea level is determined by the shape of the geoid.

oceanservice.noaa.gov

 

[RCA]

ME: Do surveyors maps often contain GIS coordinates or do they avoid using them?

ChatGPT: "Surveyors' maps typically do contain GIS (Geographic Information System) coordinates as these are fundamental for providing precise location data. GIS coordinates are essential in a variety of surveying contexts, including land development, construction, environmental management, and more. Here’s how GIS coordinates are typically utilized in surveying maps:

1. Accuracy and Precision: GIS coordinates enhance the accuracy and precision of maps. They provide a clear reference system that is essential for detailed planning and implementation of various projects.
2. Integration with Technology: Modern surveying integrates seamlessly with GIS technology, allowing for the creation of dynamic maps that can be easily updated and shared. These maps use coordinates to ensure consistency across different data sets and systems.
3. Legal and Regulatory Uses: Surveyors' maps often serve legal purposes, such as defining property boundaries and zoning areas. GIS coordinates are critical for these purposes because they provide legally recognizable and verifiable descriptions of locations.
4. Utility and Infrastructure Mapping: For the layout of utilities and infrastructure, accurate GIS coordinates ensure that the locations of underground pipes, cables, and other critical infrastructure are precisely known, which is crucial for maintenance, troubleshooting, and further development.
5. Topographical Mapping: In topographical mapping, GIS coordinates allow for the accurate depiction of the terrain, which is vital for environmental monitoring, military operations, and outdoor recreational activities like hiking and climbing.

While GIS coordinates are commonly used, there are situations where surveyors might use other types of coordinate systems depending on the specific requirements of a project or local practices. For instance:

• Local Grid Systems: For smaller, specific areas like construction sites, local grid systems might be used instead of global coordinates for simplicity and relevance to the particular project.
• Custom Coordinates Systems: In some specialized fields, such as mining or marine navigation, custom coordinate systems that are adapted to local conditions or specific operational requirements might be used.

Despite these variations, GIS coordinates are a standard component in most surveying tasks because of their versatility and the widespread use of GIS technology in spatial data management. They play a crucial role in ensuring that surveying maps are accurate, useful, and compatible with other spatial data systems."


ME: When they use GIS coordinates to the list a Geodetic Datum as reference?

ChatGPT: "Yes, when surveyors use GIS coordinates on their maps, they typically list a Geodetic Datum as a reference. This practice is essential because a geodetic datum defines the coordinate system and ellipsoidal model that describe how the coordinates on the map relate to actual locations on Earth. Without specifying the datum, the coordinates could be misinterpreted, leading to errors in location and measurements.
Here’s how and why datums are used and referenced:

1. Framework for Measurement: A geodetic datum provides the foundational framework needed to measure positions on Earth. It includes an ellipsoid (a mathematically defined surface that approximates the geoid, which represents mean sea level) and an origin point. These factors determine the numerical values of coordinates.
2. Accuracy and Consistency: Different datums can give different coordinates for the same physical location because they may use different ellipsoids or anchor points. By specifying the datum, surveyors ensure that the map’s users can accurately interpret and use the coordinates, maintaining consistency across different maps and datasets.
3. Global and Local Datums: There are global datums, like WGS 84 (World Geodetic System 1984), commonly used for worldwide applications including GPS. There are also local datums, such as NAD 83 (North American Datum 1983) in the U.S., which are tailored to give better accuracy in specific regions. Surveyors choose a datum based on the geographical area of their work and the application’s needs.
4. Transition and Updates: As technology and our understanding of the Earth's shape improve, new datums are developed. For example, the U.S. is transitioning from NAD 83 to the new NATRF2022 framework, which provides more accuracy and aligns with global positioning systems more effectively. By listing the used datum, surveyors indicate which version of geographical truth is being used, especially important during periods of transition between old and new datums.
5. Legal and Regulatory Requirements: In many jurisdictions, there are specific legal and regulatory requirements about which datum should be used for certain types of surveying work, particularly when it comes to land ownership and public works. Surveyors must adhere to these standards and clearly indicate the datum to ensure legal compliance.

Including the geodetic datum on maps and in any survey documentation is a standard practice in surveying that upholds the integrity and usability of the geographical information provided. This practice ensures that everyone from cartographers to engineers and GIS professionals can effectively communicate and utilize the data."

 

[RCA]

ME: Then is it possible to compare the referenced Geodetic Datum to a current one to see if the GIS coordinates have changed?

ChatGPT: "Yes, it is possible to compare the GIS coordinates referenced in one geodetic datum to those in another, current datum to see if there are any changes. This process is essential for ensuring data accuracy and consistency across different mapping and surveying systems, especially when updating or converting geographic data. Here's how this comparison is typically done:

1. Datum Transformation: The process involves mathematical transformations to convert coordinates from one datum to another. These transformations are necessary because different datums may use different reference ellipsoids and different origins. Tools and software used in GIS and surveying can perform these transformations based on established formulas.
2. Software and Tools: GIS software like ArcGIS, QGIS, and others have built-in tools to handle datum transformations. These tools use parameters such as translation vectors, scale factors, and rotation matrices to shift coordinates between datums accurately.
3. Checking for Shifts: By transforming the coordinates from an older datum to a current one, surveyors and GIS professionals can determine if there have been any significant shifts in position. For example, moving from NAD 27 (North American Datum of 1927) to NAD 83 might show shifts because these datums have different origins and ellipsoid models.
4. Accuracy and Precision: The precision of the transformation depends on the accuracy of the transformation parameters and the method used. For high-precision requirements, such as in engineering projects, more sophisticated transformation methods and higher-quality geospatial data are necessary.
5. Practical Applications: In practice, this comparison is often required when updating maps, integrating historical data with new surveys, or when data from different sources must be combined for analysis. For instance, a city might need to update its utility maps by integrating older surveys with new GPS data, necessitating datum transformations to ensure all data accurately represents the current landscape.
6. Continual Updates: Geodetic datums themselves may be updated or replaced as our measurement technologies improve and as the Earth's shape and the dynamics of its crust are better understood. This necessitates ongoing attention to datum specifics when working with geospatial data over time.

By comparing referenced datums with current ones, professionals can ensure that the GIS coordinates remain relevant and accurate, thereby supporting reliable decision-making based on geographical information. This process is crucial in fields ranging from urban planning and construction to environmental science and resource management."

ME: So if you are using QGIS and looking at a survey, could it transform older GIS coordinates to more accurate new ones?

ChatGPT: "Yes, QGIS can transform older GIS coordinates to more accurate new ones by using different geodetic datums. This capability is critical for maintaining the accuracy and relevance of geographic data as it is updated or integrated with new data sources. Here’s how you can perform such transformations in QGIS:

1. Coordinate Reference System (CRS) Settings: QGIS allows users to set and change the Coordinate Reference System for each project and for individual layers. You can specify both the geodetic datum and the projection system. If your data is in an older datum, you can reproject it to a newer, more accurate datum.
2. On-the-fly CRS Transformation: QGIS supports on-the-fly CRS transformation, which means it can dynamically reproject data from one CRS to another as you view it. This feature is useful for viewing and comparing data layers that are based on different datums without permanently altering the original data.
3. Using the "Reproject Layer" Tool: To permanently transform the coordinates of a layer from an older datum to a newer one, you can use the "Reproject Layer" tool found under "Processing Toolbox" in QGIS. This tool allows you to select the source and target CRS, and it will create a new layer with the coordinates transformed to the new datum.
4. Batch Processing: If you have multiple layers or large datasets, QGIS also supports batch processing for reprojection. This allows you to reproject several layers at once, saving time and ensuring consistency across your data.
5. Verifying Transformations: After transforming your data, it’s a good practice to verify the accuracy of the transformation by checking well-known reference points or integrating the transformed data with other datasets that are known to be accurate. This step ensures that the transformation parameters were correctly applied.
6. Custom Transformations: If the standard transformations do not meet your specific accuracy requirements, QGIS allows for custom transformation parameters to be defined. This is particularly useful in professional surveying and high-precision applications where default parameters may not be sufficient.

By leveraging these features in QGIS, surveyors and GIS professionals can ensure that their maps and datasets remain accurate and are compatible with modern standards, thereby supporting more informed decision-making and planning. This ability to update and transform data is crucial as our understanding of the Earth’s surface and the technologies we use to measure it continue to evolve."

 

[RCA]

Conclusion: Learn to use QGIS - and you will be able to get accurate up-to-date GIS coordinates from surveyors maps.