June 10, 2021

GeoTalks Express Working with Raster Data in Global Mapper Questions and Answers

Thank you for attending the GeoTalks Express webinar, Accessing Online Data in Global Mapper. Registered attendees for this webinar should have received an email granting access to the recorded presentation posted on our YouTube channel.

If you are interested in more detailed information on the tools in Global Mapper, including raster data analysis tools, we do offer training classes with hands-on workflows. For more information please visit our website or contact our team at training@bluemarblegeo.com.

I thought the GCP Projection pertained to the projection of the image being georeferenced. I usually set this before I digitize GCPs. 

If the image you are rectifying was generated using a specific projection, setting your workspace projection, in Configuration > Projection, to the same projection can help reduce distortion when the image is rectified. You can always reproject data by changing the workspace projection after the image is rectified. 

The Ground Control Point Projection is most important when entering ground control point coordinates or loading control points from a file. Setting this projection in accordance with the control point coordinate values will ensure they are interpreted correctly to project the image in the proper location. 

How does projection of the project come into play?

When rectifying raster data the workspace projection will be used to display the final rectified image in the Global Mapper main view. If the image you are rectifying uses a specific projection, setting the workspace projection to the same projection can help reduce distortion once the image is rectified.

​Is it better to add points first or select the projection system first?

This order of the actions, tagging control points and selecting a projection does not matter. If you have control points created and listed in the rectification dialog and you choose to change the control point projection, Global Mapper will ask if you would like to reproject or reinterpret the control point coordinates. ​

How many control points are needed in image rectification?

At least 2 points are required, but it’s recommended to use at least 4 spread evenly across the image. Start with placing a point near each corner, one in the center, and add more from there if necessary. The minimum number of points for rectification may change depending on the rectification method being used. 

How about maximum RMS error for image rectification?

In the Ground control points list, the error reports the root mean square error and the error in the X direction and Y direction, in pixels for each GCP based on the current rectification method. The default Automatic method uses all control points in the equation, and therefore does not report an error unless there are more than 3 points, and one is deselected.

Which rectification method options do we have inside this image rectifier panel?

From the Options menu in the image rectification dialog you can select to use a specific method for the rectification process. Further details on available methods can be found here in the Global Mapper knowledge base

Are there options for affine, second, or third polynomial, spline?

Rectification methods are available in the Rectification Options menu, take a look at the complete list of Rectification Methods at the knowledge base page here.

Can I type decimal coordinates in lat/long value when rectifying images?

Yes, in the Image Rectification window there is an option to type in coordinates to rectify the image.

Does this transform the raster or only shift?

The rectification transforms the data and fits it to the control points entered in the Rectification dialog. After first rectifying an image it can be adjusted by right-clicking on the layer in the control center and choosing to Rectify again. The image’s rectification is not saved permanently unless you export the file via the File > Export menu or the right click export menu from the Control Center for that layer.

​Is it possible to re-rectify the new image if the result doesn’t look good?

Yes, you can always right-click on a specific layer and try to Rectify again, or close the layer and start over.

Some images seem to drape over terrain, but I have had some that don’t drape. Possible reasons why that would be the case?

As for the imagery not draped on terrain, it would be helpful to make sure that the imagery is in the correct draw order in the Control Center so it is not lying underneath the terrain.

​Can you do a 3D (x,y,z) rectification?

If you have 3D data like a lidar point cloud, there will be an option to add Z value to a Lat Lon or X and Y point.

Did you create your own vector grid for that analysis?

Yes, the grid of vector features used for the color statistics analysis was generated in Global Mapper with the Create Regular Grid of Features tool

​​Could you describe when color statistics would be used on a project? When would you need to use it? 

We know that comparing ​the amount of land cover change between years is a fairly common one to look at. Year 1 may have a certain # of forest in a given image and Year 5 may have a different #. Depending on how you are calculating those values, you can then determine a percentage of change between those years. Measuring change is one common example for collecting statistics. 

Is your Land Use color palette available to download or is it part of Global Mapper?

The palette used for the land cover data legend is derived from the loaded​ palette layer. With a palette image loaded open Configuration > Display Options > Map Legend choose to Display Legend Based on a Color Palette. In the resulting Set up Palette dialog you can select and delete any unneeded entries to update the list. From the Set up Palette dialog you also have the option to Save Palette File so that you can use it later. 

Is there a way to show the different colors of land cover in the Control center under the layer (as ArcMap works)?

​You cannot display the colors of land cover in the Control Center, but you can view them by right clicking on the layer, and going to Options. ​Go to the Palette tab to see the different colors associated with an attribute.

In cropping, can you buffer the polygon? Say you have some lots and you want a little more than just the selected polygon(s).

Yes, you can use the Buffer tool to create a new polygon that’s larger or smaller from the original by a specified amount. The new buffer polygon can then be selected, and used to crop.  

​Does cropping affect the file size. (ie. is the data just masked?)

You would not notice any change in file size of a cropped image until you export your file either via the File > Export menu, or right clicking on the cropped layer in the Control Center. Cropping the data in the workspace technically is just masking the file as you see it, because you can turn the cropping off and the original image will appear.

Does the cropping modify the .jpg or just not show it?

If you crop an image or any raster in Global Mapper, the actual edit to the file won’t be permanent until you actually export a new image file. It’s like a preview of what it will look like if you did actually export the file.

Can you change transparency of either data set, after two images are blended together?

​You can still modify the transparency, and also other display settings, of either image file after doing a blending operation. ​

Can you specify a boundary color (e.g. black) as an outline color for vector boundaries in the raster to vector tool?

Any vector feature’s appearance can be modified, and the same goes for bounding box outlines. After creating a bounding box, you can edit that feature by right clicking on its layer and going to Options > Area Styles, then choosing Specify Style to Use When Rendering Features. Under Border Style, click Color.

Right click on the layer you want to make a boundary of in the Control Center and go to Layer > BBOX/Coverages to create a vector outline of a file.

How did you drape this?

In Global Mapper, you can drape a raster image layer over terrain by reordering the layers in the control center so that the image is displayed on top of the 3D terrain data. Open the 3D viewer and you should see the image draped on the 3D data. 

Is it possible to use normal jpg pictures (with Lat, Lon, Alt attributes) for drone ortho-photo?

When you load images taken from a drone, they will load as a Point Feature and not as an orthoimage. If you have the Lidar Module, you can select the point with the digitizer tool and right click and go to Load Selected as Orthoimages

You can use the Pixels to Points tool, to take overlapping imagery and then create a point cloud, orthoimage, or 3D mesh. You will typically get better results to place the picture points as orthos by running the Pixels to Points tool. Check out this recorded webinar introducing the Pixels to Points process

Where did your LandSat data come from?

​In the webinar, the data came from ​EarthExplorer​.​ I​n the online sources​, you can find it under Imagery ​ > Landsat8 Global Imagery (Download via EarthExplorer).

​​I have a shapefile of a location, can I import the corresponding satellite image?

You can load data from online sources based on a selected area feature. To do this, select the area feature first then open the Online Data dialog, select the source, and choose to download data within the currently selected polygon. 

Can you combine raster layers with different resolutions (cell size)?

Yes, using the file menu option to Export > Export Raster Image Format you can combine multiple layers into a single export file. The sample spacing setting in the export options dialog will default to the cell size of the highest-resolution image being exported. 

How can I save a high resolution image of a raster for a scientific paper?

When exporting data from Global Mapper you can set the sample spacing, or pixel size, for the generated image file. The default value for these export parameters will be the resolution of the image being exported. You can make the sample spacing smaller to reduce the size of the output image, but this will create some adjacent pixels of the same color as you cannot create a higher level of detail where it does not exist.

Can anything be done with pre-mosaicked images, such as Google Earth, where different dates of images have been used and freeze in the color differences at the same resolutions?

Feathering can be used to help soften the edge between any overlapping data layers. This includes layers of different resolutions and layers with different color adjustments. There are other color adjustments available in the Raster Options that may also help to balance tiles of data with offset colors. 

Why is the Export Option Don’t Write GeoTIFF Header considered “Advanced”?

The GeoTIFF header contains GeoTIFF tags storing information related to the projection of the data. This allows the file to be loaded into a GIS without any external metadata specifying the projection. The main reason a header would not be written for this format is to handle datum specializations that can’t be exactly stored in GeoTIFF files. In this case you would want to enable the creation of a projection (PRJ) file to externally store the projection parameters for the file. This use of the format and files is considered more advanced, but it does not mean that anyone can’t enable this option if desired. 

What if I have a TIFF generated by lidar that needs to be changed by 3.28 to change from meters to feet?

The vertical units for elevation data can be converted when exporting the data. In the Export Options for terrain formats select the desired units for the export file and Global Mapper will appropriately convert the values. 

What are the tiling options for raster data?

When exporting any data from Global Mapper, raster or vector, you have the option to tile the data to create multiple export files. All the options for creating tiles of data during export are available on the Tiling tab of the Export Options dialog. 


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