I will preface this by saying I am new to this field. I am using a Trimble geo7x with a zephyr 3 to collect elevation points, but my data are not what I expected to see: some are negative and do not come close when I compare uncorrected or corrected data to DEM data in gis.

I used com3 for my port setting in the field, and I’m thinking this is my error. I’m set for msl, geoid 12a, and I have the appropriate antenna height.

My question is: is there any way to correct this in office? Running a differential correction in pathfinder does not get me where I need to be.

2 Answers 2


It’s unlikely to be the port setting - that would merely turn the connection on or off (so collection wouldn’t have been possible if it were the wrong port). That being said, you can check - if you used a Windows laptop - by going into device manager and checking the properties of the serial port used to plug in the receiver (you can check this in office).

When you say “set to” Geoid12A, you’ll want to ensure these were your true collection settings. If elevation was collected as Ellipsoidal (probably still the default) than negative values can be accurate in coastal and other low-lying areas. That would just require a transformation in post-processing.

You want to check the DEM too. See the metadata information (as xml or txt file) or else source information from the web. Ensure it is in the same datum and geoid as your points. Otherwise you’re not comparing apples to apples.

With this receiver and antenna you should be attaining better accuracy than almost DEM.


My DEM was not accurate enough for comparison and I didn’t consider the tide going out when I collected negative z values.. since they are under a foot I am feeling much more confident. Thank you so much for that quick response!


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