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I have about +100 points of various descriptions from an archaeological survey. They are the backlog from years of excavations BEFORE systematically using Total Station at the site. For each artifact I have the upper and lower elevation data, in addition to the distance from either the NE, SE, SW, or NW point of the grid square. The location was recorded using two meter tapes from two points intersecting at the point of the artifact. The Excel file I have records the grid corner point and the distance from that corner to the artifact (ID1 and Radius1, ID2 and Radius2).

First, I need a method that will help me systematically upload the points. How do I arrange my Excel file, and what do I do in QGIS to get it plotted properly?

Second, I want to be able to answer questions of spatial relations based on elevations.Is there something special that I need to do for the input of the data in the .csv file?

I'm new to QGIS.

PolyGeo
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JPDA
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1 Answers1

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Your problem looks like a bilateration issue.

This way it works for me:

We start of with the corner coodinates in a projected CRS (here: EPSG:31466):

file corners.csv

corner,E,N
A,2530114,5725986
B,2530338,5726216
C,2530525,5726401
D,2530655,5726370
E,2530917,5726264
F,2531381,5725999
G,2531197,5725785
H,2530698,5725334

and the points and measured distances, one line for every distance and two for every point:

file pointdist.csv

point,corner,dist
1,A,146
1,B,211
2,B,134
2,C,158
3,A,369
3,H,516
4,A,425
4,B,303
5,D,192
5,E,120
6,G,203
6,H,563
7,H,125
7,G,586
8,E,216
8,F,462
9,F,163
9,G,166

Load corners.csv and pointdist.csv with Add Vector Layer

Rightclick on the pointdist layer Properties->Joins

Join layer: corners
Join field corner
Target field: corner
Save As... pointcornerdist.csv

Load pointcornerdist.csv as Delimited text and EPSG:31466

remove the other two csv layers

Rightclick -> Set Project CRS from Layer

Settings->Snapping options: pointcornerdist.csv 10 pixels

Create a New Shapefile Layer of type polygon EPSG:31466 name site.shp

Draw a polygon along the corner points

Rightclick on the polygon layer -> Properties->Style: Simple fill : No brush

Vector->Geoprocessing->Buffer(s)

Input vector layer: pointcornerdist
Segments: 20
Buffer distance field: dist
Output shapefile: buffers.shp
x Add result to canvas with EPSG:31466
Close

Vector->Geometry Tools -> Polygons to lines

Input Vector layer: buffers
Outpt shapefile: bufferlines.shp
x Add result to canvas with EPSG:31466
Close

Remove layer buffers

Vector->Analysis Tools-> Line intersections

Input line layer = Intersect line layer: bufferlines
Input unique ID field = Intersect unique ID fiels: point
Output Shapefile: intersect.shp
x Add result to canvas with EPSG:31466
Close

Rightclick on intersect.shp -> Filter

Click on point_1 = and point_2

Vector->Geoprocessing Tools -> Clip

 Input vector layer: intersect
 Clip layer: site
 Output shapefile: points.shp
 x Add result to canvas with EPSG:31466
 Close

Re-Add corners.csv, this time as Delimited text and EPSG:31466

label points layer with point_1, and corners layer with corner

and you are done: enter image description here

AndreJ
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  • Thanks Andre for your quick reply. Yes, someone suggested this, but what you're suggesting seems like I would have to do this for each point. Is there a systematic method I can apply to the whole dataset? If so, can you please include a specific directive? Thank you! – JPDA Sep 16 '14 at 12:16
  • Unfortunately, your points may have been measured to any two of the four edge points. If you group them by common reference points, you could try to do the trilateration mathematically. See http://gis.stackexchange.com/questions/48937/how-to-calculate-the-intersection-of-2-circles – AndreJ Sep 16 '14 at 12:51
  • Hi Andre. Thank you for following up... I don't have to guess which two edge points the measurements were taken from. Does that change the method? – JPDA Sep 16 '14 at 13:14
  • No. The only point of guessing is if both possible intersections are within your research area. – AndreJ Sep 16 '14 at 13:17
  • When you say "group them by common reference points", do you mean that I should create a separate csv file for the artifacts that are measured from the same two points, and then load that csv file? – JPDA Sep 16 '14 at 13:20
  • Hi @Andre Joost, I read through the link you provided and have done a few searches in the manual for trilateration (it doesn't show up at all). I've also now been looking for how to draw the buffers and am not making much success. Would you mind giving me some more step-by-step instructions? – JPDA Sep 16 '14 at 13:32
  • See my extended answer. – AndreJ Sep 26 '14 at 16:09
  • Andre, First off, wow, thank you soo much for the details. I've hit a snag at the "Draw a polygon along the corner points." I know this sounds funny, but I can't seem to do this. I've looked up lots of help docs on "drawing" and can't seem to figure out what is going wrong. At the moment, I don't have any tool selected, just the cross-hairs and my points from the joined shape file are in the bottom right corner of my map. Whenever I try to move the view so I can have my points centered then it snaps back to the right bottom corner of the viewing window. Help please? Many thanks, Jessie – JPDA Oct 27 '14 at 09:57
  • For the drawing: Create a new shapefile Layer, type polygon, select the layer, then switch that layer into editing mode with the yellow pencil, and create a new object with the second icon to the right, click on the corner points one after another clockwise, then rightclick to end the polygon, save editing with the disc symbol next to the pencil and leave edit mode by clicking on the pencil. For the points, click on the point layer in Table of content, rightlick -> Zoom to Layer extent. – AndreJ Oct 27 '14 at 10:22
  • Andre, Thank you for your patient answers. I was able to get the polygon to work and have followed all of your instructions. But for everything I've done (and I've done it 3 times now) I don't get the results that I wanted. What I expected would be the circles that would intersect according to the distances. Then I would place a new point at the intersection, and those would be the features I'm trying to mark without measuring each one. – JPDA Oct 29 '14 at 08:24
  • The result that I DID get was a red polygon that becomes smaller and larger based on the zoom factor, and I can see only one circle around each of the polygon outline points when I click on that specific point. Additionally, my base-map seems to have disappeared. It's listed in the contents frame on the left, but I can't seem to find it anywhere in the viewer. I'm not sure what to do next. Many many thanks for any insight and have a good morning, Jessie – JPDA Oct 29 '14 at 08:26
  • Eureka! I closed the program and reopened the project. All of the circles are there! Is it a common problem that QGis needs to be refreshed after a certain number of actions? I will update as needed. Thank you! – JPDA Oct 29 '14 at 08:34
  • Sometimes QGIS needs to be forced to refresh. After leaving editing mode, just pan the canvas a bit, or zoom to extent to force it. – AndreJ Oct 29 '14 at 10:59