Word of the Month - Ground Control Points, Part II

In this edition of WOM, we continue our discussion from last month on ground control points (GCPs) with a focus on tips to improve the collection of this data specifically for use in the orthorectification of high-resolution satellite imagery (e.g. IKONOS, GeoEye-1, QuickBird and WorldView-1/2). Having completed a plethora of orthorectification projects over the last half decade at eMap, these tips are a combination of lessons learned from the field as well as lessons learned during the production process.

As a quick recap of last month’s WOM, GCPs are used in the orthorectification process along with a digital elevation model to improve the horizontal accuracy of raster imagery datasets. There are two broad categories of GCPs: targeted (or photogrammetric) and photo-identifiable control (PIC). PIC is by the far the most common control used to orthorectify high-resolution satellite imagery as it can be collected after the imagery is obtained. Many eMap clients have asked for tips to improve the collection of PIC (specifically for orthorectification of high-resolution satellite imagery), so here we go!

(1) Selecting PIC before you go out in the field – the first step of obtaining PIC is the identification of the locations you will visit in the field. In many ways, this is the most crucial step of the process as poorly selected points can lead to extra time working in the field and/or inferior accuracies after ortho-production.

1. When selecting the location of PIC, pick out areas that look to be relatively permanent on the ground. Remember that things change so that the older your imagery is, the higher the chance you are identifying features that will not still be on the ground. As such, eMap prefers to pick PIC from areas that are in established residential neighborhoods or commercial developments.
2. When it comes to the spacing and number of PIC needed to produce a high-quality ortho, there are many factors to consider. As such, there is no set equation we can offer you to determine the spacing and number required; what we can offer however is a list of suggestions that need to be considered in light of your specific project requirements.
• If you need a more accurate product, then error on the side of collection more control than you need versus less. It is not uncommon to throw out specific PIC once you start the orthorectification process for inferior accuracy.
• We prefer to obtain at bare minimum 5 points per area so that we can throw out control that is not necessary.
• As a very high-level rule, we recommend one PIC for every 20 to 40 square kilometers.
• For long pipeline or narrow corridors, we typically collect one PIC every 10 km of linear distance.
• Distribute the locations of PIC that you pick at the edges and center of your data.
• Try to pick out a balanced mix of PIC that are at high, medium and low elevations with respect to the overall project area.
• It is always a good idea to pick PIC where various dates of satellite imagery overlap each other.
• If the topography is extremely rugged, you will need more control than in areas with minimal relief.
3. In terms of the specific locations of PIC, you need to select points that are clearly defined in the imagery and unambiguous so that when you apply this control in the orthorectification process it is very easy to identify the PIC.
• Our preferred locations in an urban environment include: light/power poles (for 50-cm resolution satellite imagery); thick wooden fence posts; lines on a tennis court; ends of parking space lines; crosswalks; railroad crossings; sidewalk intersections; and edges where light and dark pavement meet up.
• When working in rural areas, finding good PIC can be a challenge and you should expect worse accuracies given the lack of highly visible locations. That said, our suggestions in rural environments include: isolated rocks; trail intersections; bridges; fence posts; tree trunks of trees without leaves; and light/power poles.
• Be sure to pick places that appear to have public access and are safe for people to stand in the field and collect the PIC. Also, be aware that elementary, middle and high schools are not good choices for PIC locations as with the increased security of late, you are unlikely to be given access to their grounds.

1. When you are in the field, you will need a way to orient yourself to find the PIC locations you picked out. For this, we suggest brining a laptop with a geospatial application and the high-resolution imagery loaded on it. That way you can shift locations of the PIC you picked before you went to the field if there is a need to do such. An alternative method is to bring a large chip of the imagery around the locations of PIC you decided on as a JPEG, PNG, etc.
2. Make sure you bring the right equipment with you. We suggest a high-quality differential GPS unit from a reputable company. You should also bring a GPS tripod with you that has a known height and is very stable.

1. If possible, every point you collect should be done with the same GPS unit on the same tripod with a known height.
2. Once you have a good GPS reading, be sure to record the X, Y and Z positions. Double check the values you record to be sure they are accurate. You also need to record the datum, project, zone and units used to collect the PIC. Here at eMap we suggest using a UTM WGS84 projection/datum as this is by far the most accurate combination currently available.
3. Take multiple pictures of the GPS unit on the tripod so that you can be sure to relate the XYZ information you record in the field to the high-resolution satellite imagery once you are back in the office and have started the orthorectification process.
4. We recommend that you collect PIC with a positional accuracy that is at least twice as accurate as the pixel size of the high-resolution data you are working with.
5. As a final resort, if you are unable to collect the PIC you need when you are out in the field, you can extract PIC from orthoimagery that already exists (such as NAIP or a free statewide imagery layer). This is definitely not as accurate as the process detailed above, but sometimes you have to make the best of a bad situation! If you have to do this, often called second generation orthorectification, be sure to choose the bottom of features to avoid the parallax issue (or lien) that occurs in high-resolution imagery. As such, be sure to pick the bottom of a house as opposed to the roof as the roof liens in a particular direction so that the coordinate you are reading would include this lien; while in real-life houses do not lien.

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