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Question:
My DEM is currently in the NAD 83 UTM zone 16 projection which is the correct zone for our area and matches the projection of the data downloaded from the ACPF website. It says the vertical accuracy is 6.24 cm and the horizontal accuracy is 0.6 m. Should I resample my raster to have 7cm z-factor and 1-meter cell size?

Answer:
Before resampling, take a look at your settings. In ArcMap in the table of contents, right click on the DTM/DEM you are using and select properties. Click on the Source tab and email back with the following information.

  • Cell Size
  • Spatial Reference
  • Linear Unit
Also, within the table of contents, can you share the high and low values of your DTM/DEM? This will help us determine if the units are in meters or feet. If you are using UTM Zone 16, then your horizontal units should be in meters. The cell size will tell us if it is a 1-meter cell or 3-meter cell or another cell size. If you are talking about vertical accuracy, that relates more to the amount of data collected when the LiDAR is flown, not to the cell sizes. Please share the above information and that will help us with your questions. Also - You are correct on the projection information. However, the x,y units and z-units of your DEM are not the same as horizontal and vertical accuracy. If you are in a UTM projection, your x,y units are likely in meters. If the horizontal accuracy of your data is .6 m, you are “able” to resample your DEM to a 1-meter resolution. However, we often find that this is too detailed, and would suggest you resample to a 2-meter DEM. Once you do this, you will go into the properties of your raster, and the cellsize should read 2,2. Now for the z-units. You need to know what units (feet, meter, or cm) your DEM z-values are currently in. This information is contained in the metadata from wherever you obtained the data, and is NOT the same as vertical accuracy. Like previously mentioned, you can get at this information by looking at the range of values in your DEM. If your z-units are in meters, a single grid cell may have a value of 246.57, which means the elevation at that cell is 246.57 meters. What I mean by converting the z-unit to cm integer is to take the entire raster and multiply it by 100 and convert to integer. It will look like this in raster calculator: Int (inDEM * 100.00). In the output raster, the 246.57 value will now be 24657 cm. The above formula is only if your z-units are in meters. If tey are in feet, you would multiply by 30.48 rather than 100.

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