How to Compute Orthometric Heights Using ArcGIS Pro & NGS GEOID Tool

This document will describe the process for computing ortho heights for a layer captured in Field Maps. This is useful if Eos Tools Pro wasn't setup correctly at the time of data collection.

Requirements:

• Feature layer with GNSS attribute data

• ArcGIS Pro

• NGS GEOID18 Tool[https://geodesy.noaa.gov/GEOID/GEOID18/]

• Microsoft Excel

Export the Layer from ArcGIS Pro to Excel

1. Start the Table to Excel tool in ArcGIS Pro[https://pro.arcgis.com/en/pro-app/latest/tool-reference/conversion/table-to-excel.htm] on the data layer.

2. Select the feature layer as the input table.

3. Click Run

Format Excel Sheet to CSV

1. Delete all columns except for

   • OBJECTID

   • ESRIGNSS_LATITUDE

   • ESRIGNSS_LONGITUDE
     
     Example:

   OBJECTID	ESRIGNSS_LATITUDE	ESRIGNSS_LONGITUDE
   1	36.90706608	-81.28696512

2. Reorder the Columns

   • ESRIGNSS_LATITUDE

   • ESRIGNSS_LONGITUDE

   • OBJECTID
     
     Example:

   ESRIGNSS_LATITUDE	ESRIGNSS_LONGITUDE	OBJECTID
   36.90706608	-81.28696512	1

3. Delete Header Column.
   
   Example:

   36.90706608	-81.28696512	1

4. Remove the negative sign from the longitude values. NGS assumes all longitude values as West. Use the replace tool to replace all "-" with "".

5. Format latitude and longitude columns as:

   • Number

   • 8 decimal places

6. Save Excel File as TXT

   • File -> Save As -> Tab delimited Text (.txt)

Convert TXT to Free Format Type 2

1. Open the TXT file in a text editor.

2. Convert the first tab to a space and shift the OBJECTID values to at least the 33rd column 36.90706608    81.28696512    1
   to
   36.90706608 81.28696512                 1

3. Save the file.

NGS GEOID18 Processing Tool

1. Go to https://geodesy.noaa.gov/GEOID/GEOID18/computation.html

2. Scroll down to the Compute GEOID18 geoid heights for a file of input points section.

3. Click Choose File and select the free format file create in the last step.

4. Click the button to process the file.

5. Copy all of the output and paste it into a new file.

6. Save the new file as a .txt file. (ngs_output.txt)

7. Open Excel.

8. Click File -> Import -> Text File.

9. Select the ngs_output.txt file.

10. Leave all settings default. Click next.

11. Click Finish and place the data in a new sheet.

12. Delete columns A-F

13. Add a header row with the format
    GEOID_VALUE,NA,OBJECTID

14. Save the file as a CSV. (GEOID_Values.csv)

Load the file into ArcGIS Pro

1. Go back to ArcGIS Pro.

2. Run the Join Field tool.

3. Select your layer as the Input Table.

4. Select OBJECTID as the Input Join Field.

5. Click the browse file button next to the Join Table input.

6. Select the GEOID_Values.csv file.

7. Select OBJECTID as the Join Table Field.

8. Select GEOID_VALUE in the transfer fields list.

9. Click run.

Calculate Ortho Height

1. Run the Calculate Field tool.

2. Select your layer as the Input Table.

3. Select an existing field or enter a name for a new field to contain the ortho height.

4. Enter an expression to subtract the GEOID_VALUE field from the ESRIGNSS_ALTITUDE field.

   !ESRIGNSS_ALTITUDE!-!GEOID_VALUE!

5. Click Run.

Update the feature geometry (Optional)

1. Run the Add XY Coordinates tool.

2. Select your layer as the Input Features.

3. Click the Map tab then click Add Data -> XY Point Data.

4. Select your layer as the Input Table.

5. Enter a name for the Output Feature Class.

6. Select POINT_X for the X Field.

7. Select POINT_Y for the Y Field.

8. Select the field containing the orthometric height for the Z Field.

9. Click the drop down for the coordinate system selection and select your layer. This will match the output coordinate system to the current coordinate system of the layer.