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I am using QGIS and have 2 shapefiles related to a particular city: The city blocks and the census tracks. When I bring them both into QGIS and overlay them, it's clear that the boundaries should perfectly align, but for some reason the boundaries are off by a few meters: Where the thin black lines are the census track boundaries and the grey lines underneath are the city blocks. I think that the issue might be the coordinate system, but am fairly new to GIS and don't know how to manually set the

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coordinate system such that the maps overlap perfectly.

Could someone provide a easy explanation of how to fix the misalignment?

EDIT: The current Spatial Reference System for the 2-layers (taken from the metadata) are:

City Blocks: +proj=tmerc +lat_0=0 +lon_0=-60 +k=0.999996 +x_0=400000 +y_0=5000000 +ellps=aust_SA +towgs84=-57,1,-41,0,0,0,0 +units=m +no_defs

CRS_blocks

Census Tracts: +proj=longlat +ellps=GRS80 +no_defs

CRS_census

Obie K
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    This question has a lot of good information on this topic:https://gis.stackexchange.com/questions/20572/layers-with-same-coordinate-system-should-align-overlap-in-arcmap-but-do-not. It's meant for ArcGIS users though. Your two layers could be in different coordinate systems, or they could be in the same coordinate system and one (or both) of them are just not accurate. – Dan C Jan 10 '18 at 21:04
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    Please [Edit] your question to specify what the coordinate system values are for the two shapefiles. Part of it could be datum shift, but more than that is involved. – Vince Jan 10 '18 at 21:06
  • Are the shapefiles public? – user30184 Jan 10 '18 at 21:41
  • Have you set project on the fly in QGIS? https://gis.stackexchange.com/questions/10087/how-to-enable-projection-transformation-in-qgis (more help at http://www.qgistutorials.com/en/docs/working_with_projections.html) Are the two coordinate systems comparable? Not all are and need a special transformation to align, as Vince has already asked, what are the two shapefiles' coordinate systems? A screen shot of each layers' property page, CRS tab would help. – Michael Stimson Jan 10 '18 at 23:59
  • Thanks for the help. Unfortunately the shapefiles aren't public. I've edited the question to include information on the CRS for each shapefile and indeed they aren't the same. Is there a simple way to convert on to the other? – Obie K Jan 11 '18 at 01:37

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According to https://wiki.osgeo.org/wiki/Brazilian_Coordinate_Reference_Systems your data seems to be based on the SAD69 datum, but has outdated +towgs84 parameters.

You can create a custom CRS with the correct parameters according to the article, assign it to your data, save it to a new file with that CRS and the shift will minimize.

AndreJ
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