DEM general questions, raster data symbology, DEM symbology to shapefile

[SydWilso]

I'm using ArcMap 10 and have 1 arc-second, 1/3 arc-second, and 1/9 arc-second DEMs. I understand that 1/9 arc-second is about 3 meters, 1/3 arc-second is about 10 meters, and 1 arc-second is about 30 meters (http://ned.usgs.gov/).

1. This means 3 m by 3 m cell, 10x10, and 30x30? (Just making sure).

2. To change the symbology of the DEM, I use the option "classified" and am separating the cell values into certain heights "100, 250, 500" meters. Am I okay in assuming that the cell values of the DEM represent heights in meters at each 10x10 cell (of whatever arc-second map I'm using) ?

3. After I change the symbology of the DEM, is there anyway to make these sub-divisions into shapefiles? Similar to the process of highlighting certain areas/lines/points in a shapefile and then exporting it to its own shapefile? I would like to do this process for the DEM with certain elevation sections (i.e. 100-250 meters, 250-500 meters). Is this possible?

Thank you. I'm pretty new to ArcGIS.

[JimCousins]

1. Yes, that is correct

2. Yes, height is in meters referenced to the source datum.

3. If you have access to either 3D analyst or spatial analysis extentions, there is a surface toolbox and one of the tools is Contour.
   You can choose the contour interval, conversion factor (if you want to change from meters to feet) and what elevation to start at.
   Output is a shapefile with the elevation value added to each line as an attribute.

Best Regards,
Jim

[XanderBakker]

I'm using ArcMap 10 and have 1 arc-second, 1/3 arc-second, and 1/9 arc-second DEMs. I understand that 1/9 arc-second is about 3 meters, 1/3 arc-second is about 10 meters, and 1 arc-second is about 30 meters (http://ned.usgs.gov/).

1. This means 3 m by 3 m cell, 10x10, and 30x30? (Just making sure).

2. To change the symbology of the DEM, I use the option "classified" and am separating the cell values into certain heights "100, 250, 500" meters. Am I okay in assuming that the cell values of the DEM represent heights in meters at each 10x10 cell (of whatever arc-second map I'm using) ?

3. After I change the symbology of the DEM, is there anyway to make these sub-divisions into shapefiles? Similar to the process of highlighting certain areas/lines/points in a shapefile and then exporting it to its own shapefile? I would like to do this process for the DEM with certain elevation sections (i.e. 100-250 meters, 250-500 meters). Is this possible?

Thank you. I'm pretty new to ArcGIS.

1. Yes

2. All elevation values are in meters (the data is distributed in geographic coordinates in units of decimal degrees, so your cells will have very small values, since they are decimal degrees). You can project the raster using the "Project Raster (Data Management)" tool.

3. there are a few steps required:

• The symbology does not affect the values in the raster. Before you can convert them to shapefile you will first have to use Reclassify them through the "Reclassify (Spatial Analyst)" tool. Please note that this requires the Spatial Analyst extension. If you select the DEM with the classified symbology the ranges will already be specified. The result will be a raster holding a unique value for each class you defined.


• The next step is to convert this raster to polygons. The resulting featureclass will have a field called "gridcode". This holds the unique value of your raster. A value 1 will refer to your first class, and so on...

Kind regards,

Xander

I just noticed that Jim already posted useful info...

[KaniaNastiti]

Thank you for the posting.

I would like to ask some questions regarding this topic.

I want to resample the map.

I use 90m SRTM

For 90m SRTM the cell size is 0.00083333

My question are :

1. What is the best cell sizes of SRTM 250 m and 1 km?

2. How can we define of the sizes? Is there any calculation method?

Thank you for your kindness to read my problem in GIS.

Best Regards,
Kania

[JeffreySwain]

The cell size you are looking at Kania is in decimal degrees since the projection is WGS 1984.  The Wikipedia page for decimal degrees has a chart that gives you an idea of the conversion, but also remember that the conversion from decimal degrees to meters is different depending on what latitude you are. For the kind of analysis you are looking to do, I would consider converting the WGS 1984 into a projected coordinate system with meters and then resample to more logical units.

[curtvprice]

The posters are correct that your first step should be to project your data.

The project raster tool makes estimates an appropriate cell size based on the average within your input raster extent.  If you don't like its guess, you can rerun the tool with a specified output cell size.

It is best practice to specify the registration point and output cell size so that the projected rasters will line up with each other in the output coordinate system. This avoids unnecessary resampling later. The registration point does not need to be even close to the extent in the output coordinate system (0,0 will work). You should also use BILINEAR interpolation with elevation data when projecting.

Once your data are projected, the easiest tool to use to make polygons is Slice, to create elevation zones, followed by Raster To Polygon.

Hope this helps.