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Below image appears to indicate that the vector layer data was identified as NAD_1983_HARN_StatePlane_Florida_North_FIPS_0903_Feet, which is a well known (to QGIS) CRS. However when the layer was added it was automatically converted to WGS 84. (I presume WGS is an excellent "universal" CRS?)

Is the conversion algorithm inside QGIS to WGS just (about) as accurate to telling QGIS the specific source CRS (e.g. NAD_1983_HARN_StatePlane_Florida_North_FIPS_0903_Feet) ? Is there great advantage to specifying the CRS vs. just allowing the automatic conversion to WGS to occur?

I will likely add a OSM layer (not sure what CRS that will be) and may need to add other layers from the U.S. Census, Google Maps, etc. Wondering if I should pay close attention to the CRS specifics of each layer, or just rely on QGIS to convert to WGS and call it a day?

I understand in general the basics in 7.1 Lesson: Reprojecting and Transforming Data, but remain uncertain on the specifics.

CRS Florida vs. WGS 84

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QGIS does no automatic converting of the layer to WGS84.

It uses a project CRS, that can be different from the CRS of your layer(s). On-the-fly-reprojection allows to see layers of different CRS together.

Under Settings -> Options -> CRS you can secify the default project CRS for new projects. This is set to EPSG:4326, but you can set it to any other CRS you want.

For certain operations like clipping, it is necessary to have the layers in the same CRS. For that, you really need to reproject one layer to a different name and CRS to match the CRS of the other layer. This is done with Rightclick -> Save As ...for vector data, and Raster -> Projections -> Warp for raster data. In this case, I suggest to turn on-the-fly-reprojection off to see that the layers really align.

The Openlayers plugin for the Openstreetmap background uses EPSG:3857, which looks less distorted in some parts of the world. Alternatively, you can use the QuickMapServices plugin, which serves OSM under other CRS as well.

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This will depend on what are you using the data for.

If it is just generalized mapping, then allowing the reprojection to WGS84 would not create too much of an issue, if you want that for your default projection.

Reprojecting NAD83 HARN data can result in positional errors of up to 1.5 meters depending on location.

There is a shift from NAD83 HARN data to typical NAD83 data. Typical, or simple NAD83 data matches WGS84 out to about the sixth decimal place when the state plane coordinates are convert to geographic positions, if I remember correctly.

The methods used to reproject the data within Proj4 may not be rigorous and could simply take the HARN data, assume it is simple NAD83, and reproject the data without applying the necessary shift from NAD83 HARN, to simple NAD83.

These shift values can be very minor, but they can make a difference depending upon your location in the State Plane Coordinate Zone.

Personally speaking, I would set my project defaults to the projection of the vector file. It allows you to create proper buffers, spatial queries, area calculations, and other items in a measurement system coincidental with the vector file.

The files you import after that would then be reprojected to that of the vector file if you apply those settings.

Typically, vector files are much more accurate for measurements, and positioning, than imagery files.

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