7 replies to this topic

[Kristin]

I am fairly new to GIS and have somehow survived using ArcGIS thus far without a real thorough understanding of coordinate systems and how to fix projection problems.

I am working through a problem right now trying to use Google Earth images in ArcMap. The process I'm using is to georefernce the Google Earth image by marking points and obtaining known coordinates from Google Earth. As far as I know Google Earth works in WGS 1984, and when working through georeferencing I have my map "Data Frame Properties" set to this coordinate system (Necessary right?). The problem come in when I want to re-project my new georeferenced image to NAD83 which I've used for the rest of my data. My image gets reprojected off of the screen (What could I be missing?!). However if I choose to completely ignore that my new image is in WGS 1984, it does match perfectly with my NAD83 files.

I feel like I must be missing something obvious about this whole process in changing coordinate systems.

Thanks in advance for your help/advice!

[David Medeiros]

I am fairly new to GIS and have somehow survived using ArcGIS thus far without a real thorough understanding of coordinate systems and how to fix projection problems.

I am working through a problem right now trying to use Google Earth images in ArcMap. The process I'm using is to georefernce the Google Earth image by marking points and obtaining known coordinates from Google Earth. As far as I know Google Earth works in WGS 1984, and when working through georeferencing I have my map "Data Frame Properties" set to this coordinate system (Necessary right?). The problem come in when I want to re-project my new georeferenced image to NAD83 which I've used for the rest of my data. My image gets reprojected off of the screen (What could I be missing?!). However if I choose to completely ignore that my new image is in WGS 1984, it does match perfectly with my NAD83 files.

I feel like I must be missing something obvious about this whole process in changing coordinate systems.

Thanks in advance for your help/advice!

Are you re-projecting the WGS84 GE image while in a NAD83 data frame? I've found that Arc often reacts like this when I attempt to re-project within the destination coordinate system. I get around the issue by re-projecting in ArcCatalog or in an empty arc map doc before adding to the map doc with the destination coordinate system. Let me know if that helps. If not you may have an issue with the particular coordinates you are entering for the georef transformation.


Edited to add: and just to cover the obvious you are using the "Project" tool under Data Management and not "Define Projection", right?

[jrat]

I think that Google uses web mercator to display their tiles on your screen. Are you using screen shots a s your source material? If so you should georeference to web mercator. Also once you rectify your georeferenced image, Arcmap will reproject it on the fly when you add it to a map that is in a different projection.

[David Medeiros]

Google Maps is web mercator but Google Earth should be WGS84.

[dav]

hey,

just a little tip: georeferencing gearth images is very painful. you can use a download program, which delivers already georeferenced files (however I'm not sure if this is legal). for example Mobile Atlas Creator (very popular Open Source software) is very easy to use (but you will have to use the 1.8 version, because the product changed in 1.9 dramatically not allowing to download maps from gearth and other services any more but only openstreetmap (google "mobac 1.8" for finding an appropriate version). In the past I also used GmapMaker.

MOBAC delivers the data in different formats. The most handy is probably the Ozy.map data format. I don't know if arcGIS can read it. But you can try other ones. If you are a newbie to GIS, you should probably check out Global Mapper (just use a trial license in the beginning: http://www.bluemarbl.../global-mapper/ ) Global Mapper is the most easy to use GIS software for data reading and export and projection (or cropping and mosaicing....just everything that has to do with data management and preparation), and it is fast. It takes perhaps 1 hour of introduction and you will love it!

For getting images out of google earth, this is my workflow: use MOBAC, safe as png & .map ; load .map data into Global Mapper , change projection easily with 2 clicks, export data in .geotiff. ..so easy ;-)

dav

[francois1452]

You can try this:

http://www.softpedia...ownloader.shtml

It dowloads automatically images from google and georeference them. (tested with global mapper, that's ok!!)

enjoy!!

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[P.Raposo]

If it's not important that you actually use Google's imagery, one easy way to avoid needing to georectify is to simply use Esri's imagery basemap. In ArcMap 10, you can add a basemap with the arrow drop down on the add data button. Or, you can download the lyr at http://www.arcgis.co...f6a7f08febac2a9 (i.e., "Open in ArcGIS for Desktop), and add it to your map. The imagery downloads from Esri into ArcMap just like Google's, and I believe it reprojects itself into whatever your dataframe is in. Avoids the human error and imprecision possible in georectifying it yourself.

[greg585]

If it's not important that you actually use Google's imagery, one easy way to avoid needing to georectify is to simply use Esri's imagery basemap. In ArcMap 10, you can add a basemap with the arrow drop down on the add data button. Or, you can download the lyr at http://www.arcgis.co...f6a7f08febac2a9 (i.e., "Open in ArcGIS for Desktop), and add it to your map. The imagery downloads from Esri into ArcMap just like Google's, and I believe it reprojects itself into whatever your dataframe is in. Avoids the human error and imprecision possible in georectifying it yourself.

Yeah totally agree although in some areas the Google Maps are of better quality (opposite way round in some areas too).

Ill maybe have a go with francois software.