2

QGIS Version 2.18.7 and ArcGIS 10.2

Working on a Project in Northern Nicaragua and have come across some issues in projections and transformations that are a bit above my head.

Nicaragua government uses EPSG:26716 (NAD27 Zone 16N)

  • Field Data Collected using Garmin Handheld GPS64
  • Datum set to NAD27 Central, Ellipsoid: Clarke 1866

QGIS Project set to EPSG:26716, OTF transformation enabled

Within QGIS project I import a Google or Bing satellite image using Quickmapservices. Confirm the project is still in EPSG:26716. I then import a .csv of points from a legal landsurveyor performed in EPSG:32616. I specify the x/y and the coordinate system. The points plot exactly as they should in relation to the image. Now if I import a .csv of the same points collected in NAD27CENTRAL from a hand-held Garmin (supposedly EPSG:26716) and specify the coordinate system as EPSG:26716 they plot a consistent 112m south of where they should be on the image.

The difference in y coordinate value between WGS84 and NAD27 is -196m

enter image description here

enter image description here

I've included an image illustrating this shift using a nearby point in a courtyard. The red star is in the correct position relative to the image. it was collected in WGS84.

The blue star was collected in NAD27Central (EPSG26716)

If I save the blue star as a new shapefile specifying the coordinate system as EPSG:26716 the resulting nad27 coordinates in EPSG:26716 are 542063mE 1456236mN.

In Summary

EPSG32616 coordinate in field 542063mE  1456327mN
EPSG26716 coordinate in field 542053mE  1456131mN
EPSG26716 coordinate from EPSG32616 in QGIS 542063mE 1456236mN

Using ArcGIS 10.2 I can start a new project in EPSG:26716, import the survey points in 32616, specify the coordinate system then transform them using NAD_1972_To_WGS_1984_2 and the points will line up with those collected using the Garminm NAD27 projection, however, this does not line up with basemap imagery form ArcGIS Online.

How do I tackle this discrepancy?

Improve this question
  • 1
    Mayor be you are facing the problem described here: gis.stackexchange.com/questions/124322/… . Reprojecting from nad27 to wgs84 requires that you know the specific datum shift parameters for your region. Also requires to create a custom CR.In the comments there are links that can help you to get the datum shift parametets forma Nicaragua – Gerardo Jimenez Feb 12 '18 at 1:46
  • In ArcMap, are you setting the same transformation? Data frame properties > Coordinate System tab, Transformations button. – mkennedy Feb 12 '18 at 22:00
  • @mkennedy yes, using NAD_1927_To_WGS_1984_2 seems to work fine in ARCGIS, by setting the data frame transformation--thanks. Now setting a custom CRS for QGIS is proving to be another challenge. Currently I am trying: +proj=utm +zone=16 +ellps=clrk66 +towgs84=0,125,194,0,0,0,0 +units=m +no_defs – James Rogers Feb 13 '18 at 1:40
  • in QGIS using a custom CRS of: +proj=utm +zone=16 +ellps=clrk66 +towgs84=0,125,194,0,0,0,0 +units=m +no_defs The point collected in the field using garmin's NAD27CENTRAL is approximately 5m away from where it should be in the image. – James Rogers Feb 13 '18 at 2:19
  • The EPSG registry notes 10 m accuracy for the transformation, so that's what you have to live with. – AndreJ Feb 13 '18 at 11:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.