Issue with alignment of intersection points between transects and shoreline

[generoso81]

I’m using CoastSeg to extract shorelines and generate intersection points with transects.
The shorelines are correctly saved in the extracted_shorelines_lines.geojson file, and the intersection points with the transects are saved in the raw_transect_time_series_points.geojson file. When I load both files into QGIS for visualization and zoom in on the intersection points, I notice that they do not lie exactly on the shoreline. In many places, the distance between them is around 1 meter, but for some points, it is larger (2 meters or more).

Can anyone help me understand what might be causing this issue?

Thanks in advance!

[dbuscombe-usgs]

Thanks @generoso81 for this. The developer team is taking a break at the moment, and will be back in a few weeks to work on this issue. In the meantime, if you provided your ROI, shoreline, transect and any other relevant files, we can better troubleshoot the issue.

[2320sharon]

Hi @generoso81 !
Thank you for asking this question. The reason behind the difference between the extracted_shorelines_lines.geojson and the raw_transect_time_series_points.geojson is that the extracted_shorelines_lines.geojson are the raw shoreline vectors extracted from each image, while raw_transect_time_series_points.geojson are the points on each transect where a shoreline intersected it derived from the 'shore_x' and 'shore_y' columns in the raw_transect_time_series_merged.csv file. The raw_transect_time_series_points.geojson should align with the transects, but will not necessarily intersect the raw extracted shorelines vectors from the image. If you want to get a more detailed understanding how these points are created you can view the compute_intersection_QC. The output of this function the shoreline position along the transect as the distance from the origin of the transect, which is converted to an X Y coordinate later in the coastseg code and saved to raw_transect_time_series_points.geojson.

This currently is a bit confusing, so I'll be updating the documentation to explain the difference and purposes of these files a bit better.This page from the coastseg website explains a bit more about the shoreline extraction process that I'll be updating to better explain all these files.

[dbuscombe-usgs]

Hi @generoso81 please let us know if this solved your issue, so we may close this issue

[2320sharon]

Hi @generoso81

I actually ran into the issue you mentioned a few weeks ago when I loaded the geojson data into QGIS. It turns out its a visual glitch with QGIS that happens at very high zoom levels. If you were to measure the distance between the point and the link you would find it differs by a very small amount. After some digging I think this is the results of the end and start point of the transect getting slightly shifted when converted to a UTM CRS then back to epsg 4326.

Apologies for taking so long to get back to you and I hope this resolves your concern.