When generating a 3D model using photogrammetry, selecting the appropriate Coordinate Reference System (CRS) is important. The CRS determines how your model relates to real-world locations on the Earth's surface. However, mismatches can occur when the chosen CRS differs from external data or previous datasets, leading to measurement inaccuracies. This article explains the types of CRS mismatches and their potential impact on your data.
Understanding Coordinate System Mismatches
A coordinate system is not inherently wrong but may be incorrect for your specific requirements, most commonly when comparing a survey against a design surface. Mismatches can occur in three ways: differing vertical systems, differing horizontal systems, or both.
Differing vertical systems
When there are discrepancies between vertical systems, it results in a vertical shift in your model compared to external GIS data or previous datasets. Such mismatches are often caused by confusion over similarly named geoids. To ensure accurate measurements, it is crucial to use the correct vertical system that aligns with the intended reference data.
Differing horizontal systems
Choosing the wrong horizontal system can be more challenging to detect initially. Measurements such as stockpile calculations or road grade slope evaluations may appear normal. However, a horizontal shift between common features becomes noticeable compared to external design files or previous models. To prevent such mismatches, verifying that the chosen horizontal system aligns with the intended reference data is essential.
Differing horizontal and vertical systems
When both the horizontal and vertical systems do not match, significant shifts can occur between surveys. Fortunately, comprehensive quality assurance checks often detect these differences, depending on the degree of system dissimilarity. To ensure accurate and consistent measurements, it is crucial to appropriately align both horizontal and vertical systems.
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