# How to find the proper coordinate reference system with two sets of data using different measurements or too convert one system into another

I have two different entries of data in two different spread sheets. One of the spread sheets uses a coordinate reference system that I have no idea what the units of measurement are. the properties of the shape file say the crs is +proj=longlat +datum=WGS84 +no_defs, however the data points show up off in the middle of nowhere. The other set of data is properly aligned to my map and using the same crs. Both sets of data were collected from a hand held gps which i no longer have access to. Is there some way for me to convert my data all at once to realign properly with my map? Or do I have to try using a different crs? Finally if I do use a new CRS will that not allow me to merge the layers or do types of statistical analysis?

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if the data was collected from a GPS, the CRS would be WGS84 Lat/Long. Maybe try switching the lat and lon columns from your spreadsheet and see if the data lines up. – spatialthoughts Sep 20 '12 at 2:42
Could you post the coordinates for (say) four points in the dataset with the unknown CRS, together with the WGS84 lat/lon coordinates for the same four points? It might be possible to determine the unknown CRS from this data. N. – nhopton Sep 20 '12 at 8:16

You can say something about the projection just by looking at the coordinates. If your coordinate values are within +-180 and +-90 (on one and the other axis) then you are dealing with geographic non projected coordinates where units of measurement are usually decimal degrees. If your coordinates have values from 0 to a few million, or zero to around half a million, you are probably dealing with projected coordinates in meters.

Since you probably added one layer with geographic and the other with projected coordinates, this means that if no projection information is taken into account, geographic coordinates would probably be (visually positioned) near 0,0 (because they are relatively small values compared to projected, but take note they are in different units!)

In qgis for example this is what you need to do... choose a projection/datum for your project and say for example: Okay, i ll view my data in geographic coordinates (not projected - EPSG: 4326), then in the other layer properties (which uses different projection) you need to set on the fly reprojection into 4326 reference system from the local projection. To do this you have to have an idea which projection that is. This is usually related to the area/country which the data covers; countries have their own projections, and even with countries that have multiple coordinate systems (because of their size) you should be able to determine quickly which of those corresponds to your data.

Hope this helps a bit.

PS: If you are using a spreadsheet or a csv, make sure you are not making a decimal point error for example; if data was saved on a system with different locale, which uses a comma instead of a dot for decimal point for example, you may be getting your errors from there.

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Besides the coordinates there is also the Datum issue, choosing the wrong datum could lead to the "information is not where it is suppose to be" problem. Therefore, it would be helpful if you can know that also. In my country, Mexico, for instance, we have lat lon, Lambert conformal conic and UTM coordinate systems. Also INEGI (government mapping agency) uses NAD27, ITRF92 and ITRF2008 datums...

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Fist look at the coordinate systems. If there is a problem with the coordinate systems you usually will have an error of hundreds or thousands of kilometres. After that look at the datum. If there is a problem with the datum you usually will have an error of e.g. 100 or 200 meters. – Jens Sep 20 '12 at 15:04