question

Dear colleagues:
I have a simple question. I have two points with known coordinates from Egyptian Surveying Authority (ESA) forming a baseline about 1100 m and the Nile separate between them. When I observed these points by using total station (one as occupied point and the other as back sight point) I found a difference in the coordinate of the known point (back sight) about0.45 m in 2D. However, when I used GPS in static mode the coordinates are the same as the known coordinates. Which of those two methods are true and why?

mohamed abdelazeem كتب:Dear colleagues:
I have a simple question. I have two points with known coordinates from Egyptian Surveying Authority (ESA) forming a baseline about 1100 m and the Nile separate between them. When I observed these points by using total station (one as occupied point and the other as back sight point) I found a difference in the coordinate of the known point (back sight) about0.45 m in 2D. However, when I used GPS in static mode the coordinates are the same as the known coordinates. Which of those two methods are true and why?

First: what do you mean by "Nile Seperaion" ?

Second: the GPS-based coordinates are in three-dimension (3D), i.e., on the Earth surface. While, the Total-station's coordinates are in Two-Dimension (2D) which are projected on a plane surface. So, the differences are due to the map projection process. Then, the question is not "which of them is true" but it is: in which case I use each type of coordinates? There are a lot of sujects in our forum about this issue, and you are encouraged to search and read such topics.

Good Luck

Dear admin,
Thanks for your response. Nile separation means one point in the east bank of River Nile and the other in the west bank of River Nile. Second when I use GPS I used ETM map projection system and 2D coordinates ( easting and northing) so that I see that the map projection process is not the reason because I used the same the map projection system used by ESA. Therefore my question if the Nile surface bend the laser beam from total station it gave these errors?

mohamed abdelazeem كتب:Dear admin,
Therefore my question if the Nile surface bend the laser beam from total station it gave these errors?

Of course, NO ! The Nile has nothing to do with the coordinates' differences. I guess you need to read about the effect of the scale factor when combining GPS and total station observations. One of such subject is in:

https://surveying.ahlamontada.com/montada-f2/topic-t394.htm

Good Luck