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#1
GISNewbie

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Hi guys,

I have a stack og handwritten GPS coordinates - from an observer in the field - and I want to map them in ArcMap and do basic calculations. Length of transects etc. However I am stuck almost at the first step I don't know what coordinate system the lats and longs are in. I would appreciate any help, and tips to recognizing projected coordinates in the future:

An example of the coordinates are:

39.45-314 E and 148.21-635 S - Off the coast of Tasmania - I know that much.

Just putting them straight into google maps seems to give the correct locations, but I want to use them in ArcMap and use them to measure distances between points. I initially assumed they were in MinDec and just converted them to decimal degrees for mapping, but I am not sure. Please help a struggling biologist, I know this should be easy once I get the coordinates sorted in my head.

#2
tonyw

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...I don't know what coordinate system the lats and longs are in. I would appreciate any help, and tips to recognizing projected coordinates in the future:


Hi,
I use GIS in my work as a planner but I'm no GIS techie. Based on my understanding of coordinate systems, if these are indeed Lats and Long, and this is confirmed because they display in the correct locations in Google Earth/Map, then they are un-projected geographic coordinates (i.e., Lats and Long). Here is the work flow in Manifold. ArcMap should be similar.
- create a comma separated excel or text file with the coordinates, one coordinate per line and import the file into the GIS (or plot manually one by one)
- assign the projection as geographic/lats&long (not sure the terminology in ArcMap but this is the step to tell ArcMap the projection of the data)
- re-project to the projection you need.

Lats and Long are good for determining locations but are not useful for accurately measuring distances or areas. So import as lats and long then re-project before doing any measurements.

-Tony

#3
David Medeiros

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Lat Long is unprojected, it is a geographic coordinate system. You can input these directly into ArcGIS as a table of X/Y coordinates but will need to know the datum in order to correctly display them. Contact the field observer and have them determine the datum used by their GPS device.

Once you have that taken care of and the points are in Arc you will need to transform them from a geographic coordinate system to a projected one in order to do distance calculations.

These are relatively simple operations if you are familiar with the software but may be frustrating for a first time user. Do you have access to any experienced help in ArcGIS?

GIS Reference and Instruction Specialist, Stanford Geospatial Center.

 

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#4
GISNewbie

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Thanks guys, I have asked and the Data is WGS 84. However, what is the correct format for typing the coordinates into Excel. from the example:


39.45314? Or should I convert it to decimal degrees?

#5
mika

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It's good to be sure what is the notation first. Perhaps this is a DD MM.MMM notation - degrees, decimal minutes. If you are not sure just try to get this information from someone who collected the data.
I wrote a simple utility ages ago when I worked for the Museum of Zoology in Warsaw - our stuff would always collect gps readings in some crazy notations, so in order to be able to parse them all quickly we came up with this: http://webapps.carto...inateconverter/
maps made easy - www.cartomatic.pl

#6
Hans van der Maarel

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39.45-314 E and 148.21-635 S - Off the coast of Tasmania - I know that much.


I'm pretty sure E and S are flipped (so it should read 39.45-314 S and 148.21-635 E)

Could you check the rest of the logs and see if the number directly after the . (the 45 in 39.45-314) ever goes over 60? If it doesn't, it might be that those are minutes, as Mika implies. Notice my emphasis on might, because it kinda depends on the size and position of your study area.
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#7
ProMapper

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Hans, you are right there, the S or N value can not be more than 90 for any location on earth.

39.45-314 E and 148.21-635 S - Off the coast of Tasmania - I know that much.


I'm pretty sure E and S are flipped (so it should read 39.45-314 S and 148.21-635 E)

Could you check the rest of the logs and see if the number directly after the . (the 45 in 39.45-314) ever goes over 60? If it doesn't, it might be that those are minutes, as Mika implies. Notice my emphasis on might, because it kinda depends on the size and position of your study area.



#8
tonyw

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Thanks guys, I have asked and the Data is WGS 84. However, what is the correct format for typing the coordinates into Excel. from the example:

39.45314? Or should I convert it to decimal degrees?


Hi,
I'd suggest entering the coordinates as decimal degrees in Excel for importing.

It would be good to confirm the convention used to write the coordinates, and whether the decimal is a decimal or the hyphen is the decimal. 39.45-314 is that 39 deg 45.314 min? Can you pick some points for a sample, plot them up and compare to known locations? We don't know what your study entails, but if these are say confluences of creeks or street intersections, or locations of geographically identifiable features that can be corroborated in Google Earth, you should be able to tell which notation convention is correct.

What is the convention for decimal and commas of the note taker? Is it European where the comma is used where the decimal is used in North America after the units digit? 39.45 = 39,45? I'm wondering if the decimal is a really a decimal or the Deg sign in the hand written notes?

#9
GIS xtra

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Convert the HHMMSS data to decimal degree. you can use Excel to enter X and Y.
To add these data to arc map, the 'Add XY data' option can be used, while adding the data proper GCS (geographic coordinate system) should be given. These data will be converted to a point shape file.




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